Start by establishing the "T position" with your partner, meaning you are off at a right angle, with their left leg in between your legs. Next, have your partner try to throw you forward with a headlock throw (or koshi guruma if you're into judo). The general concept here is to attach yourself to your partner's right side, ultimately throwing them using their energy against them. As your partner off balances you forward, use this momentum to turn completely toward your partner, with your right foot planting either in between or just barely in front of their feet. Plant your right hand on their stomach to help elevate them, and... sit. As long as your left arm is hugging either your partner's hip or hooking their wrist with a one-on-one, you're going to have fantastic momentum.