Known as one of the best pound-per-pound grapplers in the world, Garcia provides a step by step tutorial on how its done. Demonstrating is Marcelo Garcia The Crucifix. He explains how to go to a crucifix from a side control. In this you must keep pressure on your opponent to prevent them from kneeling too fast.
Once they have their arm closest to you underneath them, you can hook your foot through their arm and go for the crucifix. The opening you are looking for is that arm to be under and supporting them, providing a hole for your foot to slip into. Of course once you have some control you will need to switch legs and stabilize your position with the throat.