12 strand? I think you mean 3 strand twisted, never heard of 12 (you only count the main strands, not the strands that make the strands). It's normally 3 or double braid for dock and anchor lines. Hmmm... they do make a full braided nylon rope (it's a real soft rope, nice to handle but tends to abraid and fuzz), maybe that's what you have? If so that would really be a challenge to do a full splice on. You probably need about 30 more fingers than you have to keep track of all those lines.
You can buy a book and then practice, practice and practice, all you need is the book and a fid. Splicing two rope together is easier than trying to do an eye but it stil ltakes some practice. Use a piece of old or cheap 3 strand to practice otherwise you may make one short rope out of two short ropes! :grin: