An omoplata can be initiated from closed guard with same side ankle and wrist control (example: you are playing guard and grab your opponent’s pants near their right ankle with your left hand, and your right hand controls their right wrist).
Once the grips and position are established, swing your leg that is positioned between your grips over the opponent’s shoulder while simultaneously pushing the opponent’s ankle away from you and simultaneously pulling the opponent’s arm open to make securing the omoplata easier.