Lirics

Did you read https://handbook.musescore.org/text/lyrics? It's pretty straightforward.

1. Click on the first note that the lyrics are to be attached to select it.
2. Press Ctrl+L.
3. Begin typing your lyrics.

A space jumps the cursor to the next note. A hyphen does the same to divide syllables:

for ex-am-ple, a syl-a-ble on one note and an-o-ther on the next

An underscore indicates that the syllable extends over multiple notes, for example when a syllable plays on the last note of a phrase and it's a long note tied into the next measure.