As you've said...easy to demonstrate in person, kind of difficult to describe...but here goes...
This way works for me:
Hold the mirror in your right (or left...whichever you like) hand.
Make a "V" with your other hand.
Bring the mirror up to your eye, and look through the hole.
Catch the reflection of the sun on your "V".
Look for a 'ball' of light on the grid, or star, or cross...whichever aiming device your mirror has.
This ball of light is your aim point.
Put down your other arm...you don't need it anymore.
Aim the ball on target, and hope they see it. If you can see the ball through your twists to keep the target in site, they'll see the flash.
Now, you don't have to go through the "make the 'V'" bit, but I find it works for me in that some mirrors I've used...well...they don't provide a crisp 'ball', and doing the 'make the V' bit makes it easier to find the aim point in the first place. Maybe my brain is hardwired to look for an aiming point with 3 reference points...who knows...but if it works, it ain't dumb!