The ways to control opponent's legs are, extending, bending, carrying by shoulder and etc.
In next case, you grip his pants bottom or knee part.
You grip his pant's bottom or knee.
In case of bottom part, you extend his legs. In case of knee part, you push his knee on tatami.
You are holding, insert under opponent's head with your left hand, then you hold him by "Yoko-shiho". Be careful for his escaping, you should hold his legs.