with frontside slappies, you put your weight on your heels to get onto the curb and then shift to your toes to lock in. with backside, you put your weight on your toes to get onto the curb and then shift to your heels to lock in. at least that's how it is theoretically.
basically, whichever side you're trying, as you're about to hit the curb, you lean your weight away from the curb so your trucks can get on and then you have to move your weight onto the curb to lock into the grind.