It is important to thread the machine in the sequence specified in the machine's manual. Otherwise, the stitch may not form properly. When it's time to change the thread, a fast and easy way is to tie the new thread to the old thread at the top and run it through the machine. When the knot reaches the needle eye, cut off the knot and rethread the needle with the new thread. It is a good idea to try threading different machines before you purchase one.
Some machines are much easier to thread than others.
Some sergers use industrial needles instead of conventional sewing machine needles. These needles are available in various sizes for sewing on different weights of fabric. These needles have a round shank, so you must be careful to insert them correctly with the long groove in the front and the scarf in the back. If you insert the needle incorrectly, stitches do not form properly. Some sergers use conventional sewing machine needles.