Part 1

In current versions of MuseScore, the behavior of the accidental buttons on the toolbar has changed, while in note input mode only. Now, instead of affecting the note you just entered, they affect the next note - so it is more consistent with how the duration buttons work. While in normal mode, the toolbar continues to affect the selected note or notes. Also, when entering accidentals from the palette, only a single-click is required, not a double-click.

Part 2 of this lesson is immediately below, and it covers the courtesy accidental plugin.

Part 2 - Add Courtesy Accidentals (Plugin)

For more information on downloading and installing plugins, see the following lesson from the Customization section:

https://school.masteringmusescore.com/courses/complete/lectures/6236650