How does one add a staff to a score?

Welcome aboard...

Here's some general info.:
https://musescore.org/en/handbook/create-new-score#create-new-score-adj…

https://musescore.org/en/handbook/create-new-score#instruments-and-voic…

https://musescore.org/en/handbook/create-new-score#add-staff

Do you need a bass clef/staff added for an instrument like a piano, and for which you already have entered the treble clef/staff?
-OR-
Do you need a bass clef/staff for a completely different instrument? Like a double bass, for example.

Please attach your score, explaining precisely what you want to add.

Regards.

While you can add staves to a score using Edit / Instruments, if you are trying to write for piano, don't start with the default one-line score and then add a staff - simply create a new score with File / New and select the Piano template. Then everything will already be set up for you correctly.

Press "i". Add the instrument you need.