First you have to understand what the cue actually is asking for... then you will inderstand which leg to use when.
I think of there being 3 distinct spots on the horses side for cues. It actually isn't that cut and dry but for now lets think of it that way. They are in fact very clost together but horses are very sensitive and know the difference. I define the 3 spots as 1)slightly forward of where your leg normally lays, 2)where your leg lays, and 3)slightly behind where your leg lays. A horse should be taught to move AWAY from the pressure of your leg.
Spot 1 controls the front part of the horse... the shoulder/front legs.
Spot 2 controls the middle part.... the barrel/rib cage
spot 3 controls the hind part.... haunch/hind legs
Spot 1 controls the direction of the horse. Most people think of the bit as doing most of the control but in fact it should be your leg doing most of the control with the bit as a reinforcement of the leg cue. Horses that are well trained can be controlled almost exclusively from your leg. So when you want your horse to turn you would put pressure on spot 1 to get your horse to move his shoulders away.... ie if you want to turn to the right then you would use your left leg to "push" his shoulders away from your leg in the right direction
Spot 2 controls lateral movement. Think of it as a kind of combination of 1 & 3. This is what you would use to ask your horse to side pass. If your horse is in an arena or round pen and is not staying on the rail... drifting into the center... you can apply pressure with your inside leg to "push" him back to the rail without actually turning.
Spot 3 controls the driving force... the hind end. The most important time to control the driving force is when its being used the most... at the lope. If the driving force is not going in the correct direction then you end up with the front end in the wrong direction and an incorrect lead. As you prepare your horse to lope ask his hind end to take half a step in the direction of the lead you want, then ask for the lope. For example, if you want the left lead, ask for half a step to the left using your right leg to push his hind end where you want it.
Of course horses never make it that easy. Just because you ask for one part of the horse to move doesnt mean the other parts stay where they are supposed to. So when I start a young horse I teach them to move all the peices independently without moveing the other parts. They learn to pivot (like reiners who spin).. they are keeping their hind end stationary while moving their shoulder. Then they learn a turn on the forehand... keeping their front legs stationary while moving their hind end and lastly how to move them together laterally.
You then teach them the same basic ideas at the walk jog and lope. Horses that learn to do that slow western pleasure lope have complete control of all their parts and are able to slow themselves down because of that control..
Hope that helps you some