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 my loop of thread is less than about 4 feet long I grab one end of the loop with the paper clip and then insert a pen or knitting needle through the other end of the loop. I tuck the ends of the pen under my two feet, hold the mixer with one hand and loosely pinch the threads, just below the paper clip, between the thumb and fore finger of the other hand. I make sure that the thread is taut and then turn the mixer on low. As the thread twists I keep the thread taut but because they will get shorter I also allow the mixer to move towards the floor a little. When I think that I have adequately twisted the cord I stop the mixer and lower it about 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.
When the cord has enough twist I stop the mixer and remove the paper clip. I grab the threads in the middle with one hand while I carefully clip the paper clip to the pen or knitting needle with the other hand. Then I have both hands free as I slowly allow the threads to twist together. After that I can tighten the the twist and then stretch the cord to encourage it to be nice and uniform.
Note I use thread loops that are about 2.5 times the length of the desired finished cord and that gives me exactly the length I need with no extra to cut away. eg. If I loop the threads around my metre stick after I twist and fold the threads my finished cord is about 40 cm. long. (If I used a yard stick it would be about 14.5 inches long.)