There are lots of tutorials on youtube regarding Flemish Twist strings and jigs.
I've made quite a few.
Nice thing is, if you screw up, you can undo one end and redo to a new length - it's a bit tedious, but not too hard. Make sure you use plenty of wax, by the way.
I tend to err on the side of slightly too long - you can always add a few more twists to shorten it a bit. This works well on a longbow. If you leave the string on the bow when you unstring, the twists you put in will stay put. If you are using a takedown, the string can untwist in storage so you may have to adjust before shooting.
Not a huge deal.
I made a serving jig out of some scrap aluminum and a nut and bolt that works perfectly.
I use B50.
Start off using two colours - way easier to keep track of things.