Here's my setup in the Mixer definition for the flaps on a glider
Note I have Flap control input in Fp1, spoiler in Fp3 and Aileron in Fp2, I don't have elevator input (snap flap), but if I wanted this then I would assign a flightphase as the switch here and then in the mixer menu define the mix % input. Hope this helps,
Note to enter the required flight phase you have to turn the big wheel when the switch line is highlighted until you get to Fp1,2,3,4.