This entry to crucifix takes place when the opponent is turtled and you are oriented to their side, hip to hip, with your near arm controlling their far hip.
From here, dig your near knee into the pocket between the opponent’s knee/thigh and their tricep/elbow. Once your knee has made it through this opening, drive into the opponent, centering your weight so that your far leg isn’t holding much weight. From here, swing your far side leg out and around the front of your opponent, making connection to your own knee with the bottom of your heel.
Once the connection is made with your knee and ankle, it is “easy” to manipulate your foot throgh and control the opponent’s near arm with your far side leg. Once you are controlling the opponent’s arm with your legs, the crucifix is mostly secured.
Depending on how your opponent is defending, it may be easiest to pull them back onto you, do a front roll over their torso, or a side roll over their head. You may also be able to manipulate them to flatten out, belly down, which is a very strong pinning position.
Additionally, you may be able to submit the opponent by manipulating their arm with your leg into an armbar or shoulder lock, or begin sinking in a variety of chokes.