Score Creating | MuseScore

If you are having trouble installing MuseScore 4 on your Chromebook, there are a few unofficial videos on YouTube. For example:
youtu.be/0ddni0u6pt4
youtu.be/xOKUpyd2DDs

There is also an out-of-date Handbook article which describes installing MuseScore 3 on a Chromebook:
musescore.org/en/handbook/3/install-chromebook

As regards starting your own score, the Handbook might help:
musescore.org/en/handbook/4
… and in particular the Basics chapter:
musescore.org/en/handbook/4/basics