Hes right, give down elevator while inverted. Although it might be a poor habit to start, you can also go into a very shallow climb immediately prior to the roll. Once you get the basic technique down you can also try using rudder during your roll. For example rolling left you would give right rudder, down elevator and then left rudder in that order.

Can I assume since you are a beginner and flying a mustang that its one of the pts mustangs?