I just discovered modal interchange and I am wondering if there is any particular method to choosing modal interchange chords, or is just sort of trial and error to see what works and sounds nice? (Apologies if this is a bit vague - please let me know if it is 
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Thank you
I’m assuming by “modal interchange”, you mean borrowed chords, a sort of mini-modulation in a progression. If that’s the case, look for examples that sound appealing to you, and then reverse-engineer what’s going on.
For instance, take the chorus for Alive by Pearl Jam, which is basically E G D A. Since this chorus is starting on E, going from E to G has a similar feel as going from E major to E minor… a G chord is like an Emin7 without the root. Even if you just look at the triads, E is E G# B, while G is G B D - there is a note in common (B), and this serves to help the borrowed chord feel more cohesive.
So you can look at this as: E major chord, modulating to G (the relative major key of E minor), and then just moving up in 5ths from there (G to D, D to A). Every chord has some note in common with the one coming next, whether it’s a borrowed chord or not (E to G has B, G to D has D, D to A has A, A to E has E). There is also a cool bit of chromaticism going on as well, with the notes G#, G, F#, A in the progression.
Hopefully some of that makes sense!
Modal interchange is just another name for modal mixture, and what modal mixture is is just when you borrow chords from a parallel key or mode and insert them into your progression.
A classic heavy metal and rock example already uses modal mixture, and that’s the i-bVII-bVI-V progression, the Andalusian Cadence.
In the key of E Minor, (since we’re guitar players, dammit) would be the chords: Em-D-C-B.
There’s three chords in that progression that are diatonic to the key of E Minor, and those are the i, bVI, and V chords. The i chord is the Tonic, the V chord is the Dominant, and the bVI is simply another tonic function chord in that key, and is thus diatonic.
The chord that isn’t, and is borrowed from E Aeolian is the D chord, since, diatonically, the seventh chord in E Minor should be a D#dim7 chord, not a D major triad.
That’s one example, and this is chord progression beloved of Iron Maiden. You can hear a clear example of this progression in their song, “Where Eagles Dare.”
Modal mixture is distinguished from a modulation because the borrowed chords are so brief that they don’t displace the Tonic chord, and may in fact serve to reinforce it.
So whether you call it modal interchange, modal mixture, or borrowed chords, it really is different names for the same thing. It’s more common than you think.
Great! Thats very interesting, thank you