I've always liked to think of each mode as defined by two key intervals.

Lydian: #4 and Maj 7
Ionian: Maj 3 and Maj 7
Mixolydian: Maj 3 and dom7
Dorian: min3 and Maj 6
Aeolian: min 3 min 6
Phrygian: min 2 and min 6
Locrian: min 2 and b5

To my ear these are the defining points in relation to the root.

There are other ways to place modes, like which chord progression clearly identifies with a mode. For example: I-IV-V is Ionian, i-IV7 is Dorian, two major chords a whole step apart? Mixolydian of the higher chord (or Lydian of the lower), i-v is Aeolian.

I watched the first part of the video, but honestly didn't feel like watching the whole thing. I found his attitude too flip, just me, I guess. Too much to do these days, but I'm glad it worked for you, Boaz

or even simpler at least for me-as the video suggests :

1 ) Ionian (no sharps ) C D E F G A B
2) Dorian (the " special") with one sharp C D E F# G A B
- - from here adding one flat for each mode keeping the previous - -
3 ) Mixolydian (1 flat) - C D E F G A Bb
4) Dorian (2 flats) - C D Eb F G A Bb
5) Aeolian (3 flats) - C D Eb F G Ab Bb
6) Phrygian (4 flats) - C Db Eb F G Ab Bb
7) Locrian (5 flats) - C Db Eb F Gb Ab Bb

This is pretty much you only have to remember, indeed. It could be a little more tricky if you need to use another major/minor scale, such as melodic minor.

As a person with learning disabilities I appreciate all approaches to learning.

In that light...

As an alternative way of looking at 'modes' & many other aspects of music that pretty much ditches 'Music Theory' terminology and instead takes a ground up, holistic, experience based approach, I find that the ideas of David Reed & Mireia Clua at ImproroviseForReal very sensible & useful.

I can honestly say that they are the lens through which I experience & try to understand how music 'works'.

There's loads of free stuff on their youtube channel too. While it's not intended as an 'introduction', maybe this one could be a useful starting point since it gives an idea of the angle they are coming from?



PS. I've picked up quiet a bit of 'Music Theory' too. The two way of looking at things are compatible (they are both trying to describe the same thing), but I find the ImproroviseForReal approach to be the one that has more appeal, & 'works' better for me.

Your Mileage May Vary...

Edited to correct typo & fix the youtube video link