Thread has a nap* and we are supposed to thread the needle with the tail that comes off the spool.
Use 18″-23″ of thread.
If you want to sew with a doubled thread:
Instead of folding the thread in half and knotting the two tails, cut a second length of thread and feed the two ends through the eye of your needle, then knot ends together. The nap of the two lengths of thread will be going the same direction and not fight each other the way the thread does when you fold it in half and knot the two ends.
*Nap: Projections on the surface of the thread in the direction which they naturally lie.