Hi deli conker
it couldn't be simpler.
If you have the model already the most practical thing is to try.
Go into elevon programming on your radio and turn the model on. Then start playing with the percentages and move the control stick around.

Different models have different control throws so i can't tell you a precise percentage.
Differential is the difference between the amount of up and of down travel of the aileron.
Keep it equal for simplicity or decrease the down travel

Davide