I'd wind the singles into balls (or, Abby's method of one ball with 2 strands, but I haven't tried that yet), starting with my felt ball cores (so the middle of the ball doesn't get all snarly), and if they are small enough, hold them in one hand, feeding the strands through my fingers.
If the balls are too large for that, I'd put them each in a jar on the floor, instead.
I feed the strands through my fingers like this: (pinkie)(finger)(yarn)(finger)(yarn)(finger)(thumb)
That way the other hand is free to spin the spindle.
This also works great on the wheel and leaves a hand free to collar cats before they get caught up in the yarn ...
When I finish one of the balls, I'd Andean ply the remains of the longer ball, so there's nothing left over once I'm done.
As far as telling how much twist is needed, let me refer you here -- this is a topic I've blogged about :-).
(posted by me this day on LJ:spinningfiber)