A lot depends on what type of person you are, how good your ear is and how you process information.
When it comes down to teaching modes we have found that all some pupils need is for us to tell them to play a C major scale over an E minor chord (arriving at the Phrygian mode) and their ear will pick up on the pseudo-flamenco characteristics of that mode without any further input.
Others need to know the maths – play the major scale a major third below the root of the minor chord you're playing over, etc. Both ways are valid, as they arrive at the same destination, but let's take a look at how the different systems work.
To make sure everyone knows the distinction between the two processes, take a look at the example below: the first scale is C major, with an E minor chord symbol, whilst the second is the E Phrygian scale - the third mode of C major.
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