When I am making a lot of cording at once my preferred tool is my hand mixer fitted with one beater and a paper clip.
If the threads are less than about 4 feet long I insert a pen at the other end. I tuck the ends of the pen under my two feet, hold the mixer so that the thread is taut and then turn the mixer on LOW. When I think that I have adequately twisted the cord I stop the mixer and lower the mixer an inch or so towards the floor. If the cord starts to double in the middle I know that I'm almost there and I pull up again and turn the mixer on for a wee bit longer.
(For threads about a yard or meter long I find that it takes about 14 seconds to get an adequate twist.)
For more information on making cording see the following tutorials
Making one or two colour cording by hand
Making one or two colour cording with a drill
Making cording using your sewing machine's bobbin winder
And for my tutorial on binding the ends of cording together at the bottom of the ornament and setting the free to become a tassel see
Twisted cord and tassel tutorial