If you are doing a Singapore twist, I think I can help, but I always do them with mono, NOT thread. First. Read/Watch this:
http://ultralightflyfishi...utube-video#.Tr_cJ2ARamc
Now if you need/want more info, here are my own insights:
Lets assume you want a finished leader six feet in length. I like the "thick end" (butt) of the leader to be about 2/3 of the total length. So in this case about 4 feet. Therefore you are going to need at least a 20 foot length of mono to start with. The video from the link above gives the general rule that the finished leader will be about "1/3 the mono length". But you can also figure how much mono you need to start with by taking your desired finished leader length, add the desired "butt" length, and times that number by two. So in this example, we have a six foot finished leader, with a four foot butt length. Six plus four equals ten. But since all of that ten feet is a double strand twisted together, we have to double this length to get the single strand length we need to begin with. Ten times two equals twenty. So twenty feet is the minimum length we need to begin with to get a six foot finished length leader with a four foot butt section. In reality this would still end up a little "short", but it's close enough. So lets begin:
Pull off 20 feet of the mono you'd like to use. (Hint: You can always use more and cut off what you don't use at the end of the process.)
Double this piece exactly in half.
Now take this piece (while holding it at the "loop" end) to someplace that will allow the two loose ends to spin freely and INDEPENDENTLY. In other words, the two ends can't touch each other. It also really helps if each loose end can remain straight and not twist up on itself. So I like to stand on the top landing of my stairs and drape the loose ends off the stairway using the balusters to keep them separated from each other. The more these ends can be straight and "hanging" unencumbered the better. But a slick floor where they can be straightened out on works as well.
Once you've got your strands in place, you are ready to begin the "twisting" by forming the loop in the end you've been holding, and allow the two strands to twist together while simultaneously spinning themselves. (The video clearly illustrates the process.) It's important to allow the strands to twist together AND spin independently as you twist them. Just remember to remove any "twisting up on itself" from each strand as you go, otherwise it will get unmanageable. You can see him doing this in the video as well, as he straightens each leg from time to time to remove the "twisting up on itself" that is occurring at the end of each strand. The longer the strand, the more of an issue this seems to be. (Using the elevated height of the stair landing and "hanging the ends" off of it, I rarely have this issue to deal with.) Continue this process until the entire "doubled" length is twisted together. (You don't have to, but at this point, I like to tie a simple overhand knot (that will eventually be cut off) just to keep the two loose ends "together".)
If you've done it right, you now have a smoothly twisted piece of mono with a "loop" on one end and two "loose strands" (and a knot if you tied one like I do) on the other. The loop end is where you will eventually add your tippet. So now you need to form a loop for the fly line. In this case, we are making a six foot leader. So measure six feet up from the loop end of this twisted mono strand, and double it over at this point. You can see now, that you are right back to the beginning of this process, except you now have a twisted piece of mono to start with instead of a single strand of mono, and the loop end "leg" is longer than the loose strands "leg". But the process here is still going to be exactly the same. So again, start twisting the two "legs" together exactly as you did previously (which will create the loop for the fly line), and continue on down to the end of the loose strands "leg". (Note: If you started with a piece of mono longer than you needed, don't go all the way to the end of this leg, but stop at the point just past your desired "butt end" length.)
Now tie a surgeons or a figure-eight knot in the whole thing (not just one of the legs) that will secure the loose strands leg at its desired length. (Which in our case should be 4 feet down the length of the leader.)
Then trim off the excess line from the loose strands leg (below the knot you just tied) and your done.
Hope that helps.
---David