Make a tillering string. Make it at least 72" long, 75" would likely be better, with a loop on only one end. Either tie a square knot on the other end to keep the strands together, or braid about 12-15" of that end of the string. Then you can use a bowyer's knot on that end to adjust the string to whatever length you want.

Here's a link on how to tie a bowyer's knot: http://www.longbowarchers.co.uk/knot.html

If you don't know how to make a flemish loop, here's a link to that, including full instructions on how to make a flemish string and your own jig: http://www3.sympatico.ca/ragiwarmbear/diy/flemish/flemish.html

I'd definitely recommend using two colors. It makes it a lot easier to keep track of your strand bundles when first learning. So order a couple of spools of B-50 material in your choice of colors. (Red B-50 is nasty stuff. The color bleeds all over everything, so I wouldn't go there.) Or, if you don't want to wait on an order to arrive, just go down to your local hobby or leatherworking store and get a spool of artificial sinew for tillering. They say it makes a good shooting string too, but I've never tried it.