For purposes of visualizing the leg cues...and the English riders may jump on me for this:
Imagine you have a tall rectangle on your computer screen. Each corner represents a foot of your horse, and you horse is facing the top. Now imagine you have tilted the rectangle to the right, so the corner closest the top is on the left.
On a left turn, you want your horse reaching furthest with his left front leg as you go thru the turn. If the rectangle is tilted right, it puts the left in front - which is what you want for a left lead.
I think the cues actually work more with the hind legs - that you are supposed to move the right leg back, not to tilt the triangle, but to cue him to start with his right rear leg and that will put him on the left lead. However, for remembering, I find the tilted rectangle easier.
Here is a link by Jane Sayoie to read & think about