TL;DR:
0. Make sure the soundfont is in your MuseScore > Soundfonts folder.
1. Open the score you want to use it in, then open View > Synthesizer.
2. Add > select your soundfont. Move the added soundfont to the bottom so it doesn't affect existing instruments. Click "save to score".
3. Open View > Mixer. If you haven't opened it before, the panel with the instruments might be collapsed at the bottom; expand it so you can see them.
4. Select one of the instruments so it turns a lighter gray than the others. Choose your sound from the drop-down menu above.
5. Every time you open the score in the future, you'll need to open the synthesizer and "load from score".
Extra note: If your instrument has multiple channels, you can set them independently by clicking the → at the top of the instrument to expand it. Likewise, if you know you want different sounds on the same staff, use an instrument preset that comes with channels (I usually use violin for just a couple or electric guitar for multiple).
Comments
Take a look at https://musescore.org/en/handbook/3/soundfonts-and-sfz-files and see if that answers your question.
TL;DR:
0. Make sure the soundfont is in your MuseScore > Soundfonts folder.
1. Open the score you want to use it in, then open View > Synthesizer.
2. Add > select your soundfont. Move the added soundfont to the bottom so it doesn't affect existing instruments. Click "save to score".
3. Open View > Mixer. If you haven't opened it before, the panel with the instruments might be collapsed at the bottom; expand it so you can see them.
4. Select one of the instruments so it turns a lighter gray than the others. Choose your sound from the drop-down menu above.
5. Every time you open the score in the future, you'll need to open the synthesizer and "load from score".
Extra note: If your instrument has multiple channels, you can set them independently by clicking the → at the top of the instrument to expand it. Likewise, if you know you want different sounds on the same staff, use an instrument preset that comes with channels (I usually use violin for just a couple or electric guitar for multiple).
In reply to TL;DR: 0. Make sure the… by Rose Egbert
Where do you access the synthesizer?
In reply to Where do you access the… by rowanclark973
Those instructions referred to version 3.x. I haven't tried in 4.x but have a look at the Handbook:
https://musescore.org/en/handbook/4/soundfonts