power drill (preferably with a chord for better consistency)
Tie the thread to a bolt, then set up a drill about 10-12 feet away.
Walk the thread from the bolt to the drill, then back again a few times (this requires experimentation, but try starting with 6 or 7 passes).
Keeping the string tight, start spinning the drill clockwise. The string should start to tighten up a bit. Keep spinning until the twisted string has shortened by a foot or two.
Keeping the string tight, fold it in half so the two ends are now joined. What used to be the center of the string should now get connected to the drill.
Reverse the drill’s spin so it goes counterclockwise, then spill the string to let it twist back on itself. The string should start to get longer again. Keep spinning until the twisted string stops lengthening and begins to shorten again.
The part nearest the drill is now where the yo-yo will go. Measure the string from there to the length you want it, tie it off and cut off the excess.