It's a pretty straightfoward procedure:
- mix the elevator stick on both channels. The value will be either 50% or -50%, depending on how your servos are set up. If the control surfaces move in the wrong direction, swap the sign.
- mix the aileron stick on both channels. The multiplex parameter will be "add", and the values will also be 50% or -50%. Proper motion: when the stick moves left, the left control surface should raise and the right control surface should lower. Vice versa for moving the stick right.
Here's some notes on how elevon mixing works:
http://hobbyrcs.blogspot.com/2011/05/v-tail-mixing.html
http://hobbyrcs.blogspot.com/2011/05/elevon-v-tail-mixing-calculations.html
Here's my mix. Yours should be the same, possibly swapping out the positive/negative values.
Ch 2 Ele 50%
+= Ail -50%
Ch 3 Ele -50%
+= Ail -50%
