Once you've cut your knee across to catch their foot, gotten your triceps and propping leg to successfully work together to keep the foot trapped, there is still the submission itself to be executed. Like virtually every submission in Brazilian Jiu-Jitsu, the set up is more than 90% of the work, and a good set up will yield a good submission most of the time. Here, there are a few smaller details that will help.

First, be sure to put the flat part of your wrist (the top of your forearm) flush with the back of their ankle, running right along the Achilles tendon, but still flat. Next up, push their shin outward with a fair amount of force; this rotational energy will be converted into foot-breaking awesomeness if you keep things tight here. Keeping the pressure on, make your figure four grip by grabbing your own wrist (your fingertips should be slightly white here, indicating a lot of built-up pressure).

Make sure to close both elbows tightly into your ribs, making sure there isn't any extra space. Plant your head (you can slow this down so your head doesn't make a loud "thud!" sound by keeping your propping leg planted for now), and then bring your feet together. The general concept is to widen your knees so that your hips can lower to the ground, creating a great deal of pressure right on the fulcrum of the lever you've created behind their ankle.