Common "flaperon mix:
1st!!! make the ailerons work normally on a Y harness. This checks that the servos are centered correctly and the linkages are correct.
One aileron's servo plugs into the normal aileron channel.
The other plugs into the other channel to be used (usually 6 or AUX1 to use pre-programmed mix capabilities)
Mix Aileron to the other channel and mix the other channel back to the aileron channel.
Adjust the mix percentages to get even travel for ailerons and flap function.
Pre-programmed flaperon mixes will have options for max aileron travel, differential aileron throw and flap range independent of the aileron travel range.
Exact instructions vary with the radio.
If the closest you can get to having them operate the way you want is correct aileron but reversed flaps (or reversed elevator when doing elevon mix) just swap which aileron servo is plugged into which channel.