Floss should be cut about 18"-20" long, or twice that if the thread will be doubled for the loop method. Some people like to use one arm length when doubling.
Metallics or any fibers with rough surfaces should be cut somewhat shorter to help prevent fraying.
Separate the floss into individual strands and then recombine them. This is known as "stripping" the floss. There is less twisting and knotting, and the stitches lie flatter. To separate a thread from the others, hold onto the top end of the thread between your thumb and forefinger. Pull down on it with the other thumb and forefinger, taking all the other threads with you. It looks like a knot will form. Have faith. Everything comes out just fine.