G'day Mate,
Ok first things first, set your wing type to "Flapperon", I'm assuming you have a JR RX, plug right aileron into "ail" slot, & left aileron into "aux1", of the RX, adjust servo direction with servo reverse function if needed. That's the ailerons taken care of.
Now for the elevators, plug right elevator into "elev" slot, & left elevator into aux2 slot, of the RX, now go to mixing, & use a trim include mix, either mix 5 or 6, see page 47 to page 49 in your manual.
Your master channel will be Elevator & your slave will be "Aux2", set switch position to "ON" so there is no switch involved, & you can't accidently turn it off during flight, also mix will be 100% so you get even travel up & down. And you must set the throws for elevator & aux2 to the same amount, ie 100%, or 150% whatever as long as they are the same. So that the servos will match each other's throw amount.
Then READ your manual as you play with it, that's how I learnt to program my X378 & DX7, but do all this at home, where you have time, no pressure & no distractions.
Best of luck,

As for your ST90, supertigre suggest a 11x8 to a 13x7, prop range, sounds like a gutless engine to me, I'm running a 12x8 on my OS 61FX, on my Diabolo, & it screams, & that is towards the top of the ST90 prop range.