Alright, there are a few schools of thought on the way to apply it, but I'll walk you through the really basic straight achilles lock that I learned from Steve Maxwell, because it's the one that works best from the guard for me (besides the heel hook, but that's not really a basic move and you should wait a while with that one).
Actually if you wouldn't mind, that would be awesome. I have tried it before but I am definitly doing it wrong.
So, when you drop for the straight leglock from your opponent's guard, you are going to trap the leg that you are attacking between your legs, so that their thigh (ideally as high up on the thigh as you can be) is in your crotch. Your opponent's ankle should be in your armpit and under the outside arm. If you are attacking your opponents right leg, it should be under your left arm.
Your forearm should be right at the base of the achilles tendon, right above the heel. The submission will come from raw pressure on the tendon, so make sure that you are using the bone of the forearm, and not the fleshy, muscular part. Even if you've got strong forearms, it's not as effective.
Arch you back up to finish the move. You should be on the top of your back, like in a bridge, and you should be squeezing tighter as well to exert extra pressure on the tendon to cause extra pain.
Use a guillotine grip.
Pinch your knees together.
Pin your feet on the floor to give your back more leverage.
Hope that was helpful. If it is, rep it.