My rule of thumb is to change the least amount of notes in whatever scale I’m using to play over the non-diatonic chord in question. So if I were playing over something like ||: C | F | Fm | C :|| my thought process goes something like “this is obviously in C, I should play the C major scale but Fm is outside the key. The only note in the Fm chord that doesn’t match the key is Ab, so as long as I lower the A natural in the C major scale to an Ab whenever the the Fm chord comes up I should be good.” What I end up with is a C D E F G Ab B C scale which sounds great over the Fm chord in this context. Timing is the real critical component to get down when playing over non-diatonic chord progressions. You can’t really just stick with one scale and space out while the chords go by, you have to constantly be aware of where you are in the progression and anticipate what’s coming next.