First of all, it takes three times as long to splice 12-strand as regular 8-strand. With 8-strand, you have two pairs of pairs. Each pair follows its own path around the line, so you have four "tuck paths", if you will. Both strands of a pair stay together, and when you open up a hole for one and tuck it, you also tuck its partner right alongside it without making a new hole. With 12-strand, each strand follows its own path, separate from the rest, so you have 12 separate paths to tuck, giving you three times as much work and 5 times as much head-scratching and aggervation. Best thing to do is buy only 8-strand mooring lines. I don't believe, all other things equal, that 12-strand is in any way significantly stronger than 8-strand, nor is it cheaper. You WILL eventually have to splice a mooring line or two, and maybe even a dozen at once if you get blown off a dock, so may as well get the easy stuff. However, if you are reading this, that means you are stuck with some 12-strand you need to splice. If the company is willing to pay three to five times as much overtime for splicing, well, enjoy it while you can. Okay, let's get started on a splice.
First of all, get your preliminaries out of the way. tape tightly where the bitter end will be, making sure you have good, sound line from there on up, and cut the frazzly stuff off neatly. Measure up four feet or so for the part you will unlay, and tape the line. A common mistake is making the tits too short. Really, who cares if they end up being a foot too long? That is what your rope wrench is for. Better too long than too short. You want them longer than necessary so you have plenty to pull on and you can get all the tucks you think you will need. Measure off your eye (I am assuming an eye splice) and make sure it is big enough. I like to make the eye about 8 feet long. Short eyes are weak, because the pull on each side is at more of an angle. In other words, some of the pull is translated into sideways force splitting the eye at the splice. Longer is better. Both sides of the eye are pulling from nearly the same direction. Measure the eye, and mark with duct tape.
Remove the tape from the bitter end, unlay a foot or so, and tape the strands tightly. For this, electrical tape is better because it makes a nicer, smoother end and holds compression on the ends. Trust me on this. Or don't, and use duct tape, I don't care. I like to cut all 12 pieces of tape ahead of time, and then unlay and tape. Notice that half the strands are left-hand twist, and half are right-hand twist. Notice also that the left-hand twist strands travel around the line in the right-hand direction, and vice versa. Remember this little fact because it is very important.
Now it is time to orient the standing part, bight of the eye, and bitter end. I am right-handed and I prefer to have the eye off to my right, so I can drive the fid in with my right hand toward the left. The directions I give will presume you use the same orientation. also, the standing part is closer to me than the bitter end.
Begin unlaying the rest of the bitter end. You want to end up with a tapered braid and the strands coming out from it, fan-wise. You should be looking into the heart of the line. Stop just short of your mark, so if you screwed up you can unlay a little bit more. Now, if you did this correctly, you will end up with all right-hand strands on one side, lefties to the other side.
Turn the bitter end over so that the outer surface of this taper is up. Place it on top the standing part where you have marked it for the splice to begin. Remember, I am assuming that you follow the orientation I gave, which is eye to your right and standing part to your left. Half of the strands should be coming out of the fan toward you. These should all be right-hand twist. The other half should be pointing out away from you. These of course should be left hand twist.
The two strands that come out at the very end of the tapered braid are the first two to tuck. Notice which one comes out underneath the other. The underneath one will be tucked before the over one. The over/under relationship will be maintained. Select your entry point into the standing part for your first tuck. If the right-hand lay strand of the bitter end is on top of the left-hand one, your insertion point should be just above where a right-hand strand of the standing part crosses over left-hand. Or vice versa. You see, each strand of the bitter end simply follows along with its corresponding strand in the standing part, following right along with it as it goes over and under other strands, and round and round the line. The twist must match. A leftie will not be matched up with qa rightie. Okay, so tuck your first strand. It should pass under two. The first one it passes under must be the one that its opposite will follow. Don't pull this up tight. Leave about 6 inches of strand between bitter end braid and standing part. Tuck the other one, now in the same manner.
Pick the next spot down the standing part from your first insertion. This is where your third and fourth strands enter the line. Make double-dawg sure you have the right one going the right way. The strands go their appropriate directions, under two like the first pair. Pick the next crossing down the standing part for the third insertion, and tuck the third pair in the same manner. Take a break. Triple check that the strands coming toward you, the left-hand laid strands, all have right-hand twist. Vice versa for the opposite ones. I hope you remembered to not pull this up tight just yet.
You COULD continue in like manner with the fourth, fifth, and sixth pairs, each time inserting the pair in the top of the standing part and tucking right and left under two, but you would not like the result. Instead, we tuck the last three pairs all the way through the line and out the other side. Begin with the fourth pair. the insertion point is the next crossing down the line from the third insertion. Go right through the center with both, and come out at the corresponding strands on the other side. Trace the strands around to make sure you have the right strands of the standing part. Otherwise, you will end up with two tucked strands following one strand of the standing part. Double check and triple check. Leave it loose like the first three pairs. Go up one more crossing and insert the fifth pair, coming out just down the line from where the fourth pair comes out. Go down one more crossing and in likewise manner insert the sixth and final pair.
Inspect your work carefully. Follow strands around and make sure you have it right. There should be one tucked strand corresponding to each standing part strand. Each tucked strand should have the same twist as its matched standing part strand. There will be three rows of tucked strands coming out of the standing part... one row with three right-hand twist strands, one with three left-hand twist strands, and one row with three of each. Carefully and gradually pull the tucked strands tight. You are aiming for a smooth entry and transition between the eye and the standing part.
Did I mention that "down the line" means toward the bitter end, and "up the line" means up the standing part away from the bitter end? Well, I am mentioning it now. The configuration of the tucks is not symmetrical at this point. We will change that with the next half dozen tucks. The third pair of strands that you tucked, as well as the last four strands, will now be tucked, giving you three rows, each with two righties and two lefties. Begin with the last two strands that you tucked, which are coming out the central row of six strands. Hey, let's call them Eleven and Twelve. Make sure you have them both matched up with their corresponding standing part strand, with the same twist. Open up a hole alongside the standing part strand, following with it under two. Before pulling out the fid, give the strand you will tuck a few good yanks to snug it up good. Remove the fid and tuck the strand. Do its partner going the other way in the same manner. Now get rid of strands nine and ten in the same manner. You should be left with only two strands in the central row that once had six strands. The other two rows that originally had three strands coming out, now each have five strands. In order to have four in each row, both side rows must give up one each to the central row. Tuck strand five and strand six toward the central row. They should come out just up the line from the other two strands in the central row. Now everything should be symmetrical. If you want to count your tucks, call this "one".
Work your way around the line evenly, so you are always tucking under only standing part strands, and never under a tucked strand. That is just extra work, right? Before making a tuck, check and be sure that another strand will not be tucked underneath it. If another strand will later have to be tucked underneath the one you are fixing to tuck, then tuck the other one first. Always tuck over another tucked strand, never under, and this will help to keep you straight.
When you have tucked your strands out for about 2/3 their length, you can begin to taper your splice if you wish to do so. Figure how many tucks you have left in each strand. Divide the number of yarns in each strand by the number of tucks left, and cut out that many yarns, nice and close to where the strand comes out of the splice. Repeat with each strand and each tuck. Then, at the end, for the final tuck of each strand, tuck under three, or four, whichever you like, strands instead of just two. Cut it off short, and the end can safely disappear into the splice. The splice will then have a nice taper and look very neat and professional.
Remember to always yank a strand hard and get every millimeter of slack out of the previous tuck before tucking the strand again. Take your time to follow a strand's path around the line, and the path of its neighbors, to make sure you are not skipping one or doubling up. Make sure the strand you are tucking is twisted to the opposite direction to which it leads around the splice. Always make your hole plenty big before trying to tuck a strand, and if it closes up too soon, do it again. Practice makes sorta perfect. Your first splice might come out not-so-nice. You know what? It will probably still hold, and probably be stronger than the line itself. Don't sweat it. Mostly, a bad splice will simply be big, bulky, heavy, and ugly. Not a super biggie. Just keep trying to do it better, neater, and stronger, each time you splice an eye in this stuff. Okay? Okay.
If you screw up, STOP, PULL EVERYTHING OUT, AND START ALL OVER AGAIN. Maybe after you have done 20 or 30 splices in 12-strand, you will be able to straighten out minor fuckups and have the splice come out correctly, but not yet, pilgrim. Remember too, that if it looks right, it is right, and if it don't, it ain't. And also, beating on the splice just makes you look like a bozo. There is no need, unless you just have to take out your anger or something. Okay, happy splicing.