How to put my leg behind the opponent's leg on the same side to execute a throw?

Previously,  one of the problems I have had when attempting this particular kind of throw is not being able to put the leg at the right position fast enough before the opponent detects it and counter-throws instead.  Today, I realize that key is not about how fast I can put my leg at the right place, it is about locking the opponent's front foot such that he cannot move it at all.  When he can't move that foot, I can all the time I want to put it in the right place, which also requires my leg to be very deep behind and under his leg.  In order to lock his front foot, I need to create a line between his front hand and his rear foot, and then rotate his top towards his front foot. In order to put my leg deep enough behind the opponent's leg, my other foot, which is my supporting foot, need to be very close to opponent's front foot too. As I move my free leg, that support leg and my structure need to be solid and fixed, so the opponent can't stop my free leg at all. When my free leg is placed at the target position, it needs to be bent and touching the opponent leg. When that leg is straighten, the opponent's front foot support will be taken out by the leg lever, and he will be rotated and fall.