I saw a simple enough little device once for putting a loop on a plain string. Held the folded end of the string neatly in place, and you turned a little handle and it neatly twisted the string without breaking it, so you ended up with a loop of the right size.

But I've idea where such a thing could be found. Maybe someone here might have an idea.

Doing it the other way would be tricky with steel strings, though of course it's easy putting a ball on a nylon string - just hold the end in a flame for a few seconds.