If necessary when you make the step with your right foot lean into it so that most of your weight is on the right foot. Remember this is a hand throw. The arms which started by pulling forward never let up. The thrower's right leg is nothing more than a trip wire. The arms and shoulders start and finish the throw by whirling the man to the mat. If the throw is done slowly it will be difficult to get the arm action. All the actions of the arms, leg and body should blur into each other in one thunder-clap of a throw. To get the arm action right, start by moving your body into position without moving the right leg across. Practise turning the man into the mat just with your arms alone. Once you find this can be done successfully start moving your right leg across. If you find that you are having to use your right leg to sweep or hook to get your man over go back to practising with the arms alone. My teacher in Japan once described the leg action in tai-otoshi as an afterthought. The man should be almost thrown before you use the trip.