In short: know what the simple written version would sound like if played literally like that, and then do whatever sounds good to you. You must develop your own sense and taste for what sounds good through practicing and trying things. Theory gives you ideas and tools for finding things to try, and to reason about what others play.
We could solve this question either by writing an endlessly long list of logical rules and checklists that even a deaf person could mechanically apply... or we could solve it like it's actually done: with ear and taste.
The only actual rule is: play whatever supports the melody appropriately for the occasion. If it makes you feel better, play it. If it makes your audience feel better, play it. If it makes you or your co-musicians confused, don't play it.
Chord symbols are written (by people) as guidance for accompanists (other people), and together with the lead melody, they describe the lead-sheet writer's view of what the most important idea of the song is, when described in simple, coarse terms. But it's just a starting point - all that can (and should) be jazzed up in various ways. Whatever chord symbol is written, it's just one possible idea, and each chord symbol should be read as meaning a rough umbrella term as a whole family of possible chords. And even then, you can ditch the whole rough idea and play something from a different family of chords, if you think that it suits your own idea better.
Quote from Jazz Piano Book by Mark Levine
The unhappy fact that the chord symbols C, C∆, Cmaj7, CM7, C6, and C69
all mean pretty much the same thing and are often used interchangeably
can be discouraging to a beginner. In this book, I’ll use the ∆ symbol
for all major chords.
So, in his book, he writes a "maj7" chord symbol, whenever he means any sort of non-dominant major chord. The actual realization of the described idea (IF you decide to follow that idea and not some other one) can even be a simple triad, or it could be a four-note or five-note chord, and it still fits the same general category. But dominant chords are a different family of roles, as you know.
However, lead sheets are not generally written like that - Levine just wants to say with this notation that many types of chords are interchangeable, and when you see any written chord symbol, you should generalize it in your mind to a wider overall chord family, and then play something within that general ballpark. You learn to do that by trying different things and messing with the songs.
Your example lead sheet of My Favorite Things, starts with four consecutive bars of "Em". It certanly cannot mean that you must hold the exact same chord for four bars, that would be boring, wouldn't it? Myself, I would be tempted to play a descending line: Em - Em maj7 - Em7 - Em6 - ... and on the next line, instead of the written Cmaj7, I would do the same over Am: Am - Am maj7 - Am7 - Am6 (depending on the occasion, maybe trying not to sound like Stairway to Heaven). But then there would have to be yet another change on the third line to get a change there... Maybe if the second line has to be a C based chord, I might alternate between Cmaj7 and D/C, to keep some motion going.
what are my guide tones for that?
Your guide tones are from the chord that you imagine. You might see four bars of "Cmaj7" written on the lead sheet, but if you want to play: C - D/C - C - D/C, then over the imagined C your guide tones are from C, and over the imagined D/C your guide tones are from D/C.