Here's an unorthodox method of memorizing scales using the common three-note-per-string shapes we often employ during runs.
Let’s say that you wanted to learn the E minor scale (E Aeolian mode). Normally you’d learn the box positions of the scale and drill it from there. Learning a new scale using a box position is a good way of becoming familiar with the intervals involved, but we’re going to be different and move our scale onto one string.
By doing this, we take the focus away from memorizing a shape and instead focus on the sound of the scale itself, because this is what really counts when learning a scale or mode. If we haven’t internalized how it sounds, all we’re doing is trying to recall shapes that mean very little to us.
Using the high E string, move through the intervals of the scale slowly. You can start at the 12th fret and ascend to your highest fret, or start at the highest fret and move down to the 12th. For any modes in the key of E, a 24-fret guitar will help as you can start at the 24th fret and work your way down. If you have fewer than 24 frets, start at the 12th fret and work up. (For other keys, just shift your position so you can cover a whole octave on one string.)
As you do so, notice the relationship between the sound of the next interval and the distance you need to move, whether it be whole tone or semitone. If you haven’t done a lot of ear training, this is a great way to get your fingers better at predicting where to be, according to the sound you require in your head.
The next step is to break down the scale into three-note-per-string shapes. For all the modes of the major scale and of the melodic minor scale, you’ll notice the same three shapes repeating. As you’ll see in the video above, you can step down or up by playing repetitions of these shapes.
When you’re fairly confident you can move through the scale on the E string, it’s time to apply the same approach to the B string. As you’ll see, the locations of the patterns will change, but the patterns themselves are the same three shapes. These shapes can be applied all across the neck in any mode.
When you work on the modes of the harmonic minor scale, you’ll encounter two different shapes (shown in the video). However, the approach is the same. You can cycle these five shapes up and down one string until you internalize how that mode sounds and how to navigate it. After some time, the thought is taken out of the process and your fingers pick the right path according to the next interval being "dictated" internally by ear.