One of the best ways to start working on the straight-ankle lock (also sometimes simply called the "footlock") is to start from combat base, right after having opened their closed guard. This means that you will have both of your legs on the inside, an incredibly important concept for attacking legs. Start with your left knee up, and post with your right hand (make sure your post is far enough away so that your partner can't simply take it away by grabbing at it). Now that your weight is off of your right leg, retract your right foot and place it tightly on your partner's hip, toes turned out. You can also reinforce this movement by cupping your partner's knee with your left hand, helping to ensure that your partner's leg will remain trapped. Before you release your "post," be sure to overhook the ankle, very close to the heel, but not so low as to slip past the heel. In order to finish, make a tight guillotine grip first, and then try to hide your elbow underneath your body, eliminating all space. Push your hips forward and circle your shoulders back to apply the finishing submission.