Generally (a couple that I've owned), there is a push detent so that
you can remove the spool, or spool and some sort of cover. Clean the
dirt, grass, etc., out of things, so the "tap" advance will work.
Get the right size "string." The spools on the ones I have owned have
either a notch or small hole to retain one end of the string. Then,
fill the spool with string. There is usually an arrow to indicate
which way the string is to be wound.
Snap the spool/cover back in, threading the string out through the
exit port. Pull enough out -- maybe 6 -8 " to get things started.
Start trimmer. Tap on ground or other firm surface to cause the string
to feed. There is usually a knife to cut it to length