Mixes help keep the plane flying straight on knife edge passes. When you apply rudder on knife edge sometimes the plane will either pull towards the canopy or push away and mixing rudder to ail will eliminate this problem.

Some people like to mix ail to rud to coordinate turns as on a full scale aircraft.

If you are unsure on how to mix or how much to mix, talk to an experienced pilot at your club. He can help you. Ask him to fly your plane and mix what is necessary and to keep test flying it until it is good. He may not be familiar with your radio but if he had computer radio experience he will be able to figure it out and show you how he did it.