Ok, for a Sasae Tsurikomi Ashi to the lapel side you set up exactly like you would if you were throwing to the sleeve side, I'm right handed, so I usually throw to my right. Break their balance to their front left and pull them forward. When you're doing this to the collar side you will have a tendency to pull down instead of pulling them forward. You do not want their weight on that foot. Draw them forward making them take a step with their left foot. When you begin to sense them stepping block their ankle with the bottom of your foot. It's the same principle as what happens when you walk through a doorway and catch your foot on the threshold, it's the exact same fall. Their body is moving forward but because you have blocked their foot they will have nothing to put their weight on. As you are drawing them forward with the lapel and causing them to step forward, also, make sure you don't forget about the sleeve or that arm. You should be lifting that arm to help off balance and bring their upper body forward and to their left.
A rushed explanation, I'll come back with a different throw later...