Target shooting or hunting with a longbow is a popular practice around the world. Some even go so far as to make their own homemade bow, along with homemade bowstring and arrows. Even if you haven't made your own bow, you may find that purchasing bowstrings is the most expensive part of your hobby. A complete novice with the right equipment can make a serviceable bowstring, as with any craft or tool you'll find that the finished product improves with practice.
Measure and cut five or six strands of waxed nylon string that are two inches longer than your longbow.
Tie a knot at one end of the nylon strands to join them together, then clamp that knot securely into the vice.
Insert the other end of the string bundle into the power drill, making sure the tension is equal on each strand. Tighten the drill down and, keeping light tension on the string, spin the drill slowly to twist the bowstring. It should twist tight, but not so tight that it coils up when you release tension on it.
Maintain tension on the string as you release it from the power drill, then knot the loose end so that the strands can't untwist. If the string coils up you wound it too tightly--undo the end knot and then release a few of the twists and try again.
Tie a loop in each end of the string that's just large enough to fit over the end of your longbow.
Keep your string waxed in order to prolong its life.
Check your bowstring frequently for signs of unravelling or fraying--these can pose a serious hazard.