1st, make sure the rudder is centered when the transmitter and receiver are turned on. Also make sure the trailing edge of the ailerons are even with the bottom of the wing.

2nd, check the vertical stab (the part the rudder is attached to ) to insure it is aligned properly with the C/L of the plane. Run a piece of string from the center of the fuselage at the front of the plane (the forward top portion of the main wing is fine ) to the center of the fuselage at the tail. The vertical stab should line up with the string.

3rd, block the FUSELAGE so it is level (do not use the landing gear ) from side to side. Check that the main wing AND the horizontal stab (the part the elevator attaches to ) are also level.

4th, if the first 3 check out ok, check the engine. Many of the motor mounts in trainers have plates that go over the mounting lugs of the engine. If your trainer has this type of set up, make sure the engine is at 90 degrees to the firewall (the part the motor mount bolts to ). If the engine is not at 90 degrees, adjust the engine so it is 90 degrees to the firewall, if the engine is at 90 degrees to the firewall either loosen the plates and adjust the engine so the front of the crankshaft is about 3/32" - 1/8" to the right of the centerline when looking from the rear OR if your engine bolts directly into the motor mount, insert a small washer between the motor mount and the firewall at the top AND bottom of the LEFT side of the motor mount (when looking from the rear ). Your local hardware store will have a selection of different thickness of washers in the fastner section (a #6 washer should work fine with the motor mount bolt - don.t forget to put some locktite on the bolts before rebolting the motor mount in place)

Then go fly the plane after the adjustment(s) and readjust as needed. The engine will be the easiest to adjust at this point if you needed to remove/reattach the vertical stab.

This should solve the problem.