Test glide the model. Make a note of what happens. It can be very helpful to have someone film the attempts and talk about what you think while filming. Sometimes the trims will require work back home on the bench. Make notes and the effects of each trim change.
Once you have the model flying well, make notes on all angles, mark the CG on the fuselage.
Make one adjustment at a time and take note of the change in flight. The most common trims are CG (balance point), elevator trim and side thrust. If all the controls are straight and the model glides straight but pulls hard to the left under power, add more right thrust. If the model still refuses to fly straight and the right wing rises, add the suggested washout to the right wing tip. This adjustment has more effect as speed increases.
If the model pulls up too hard when given full power, try more downthrust and then perhaps less incidence angle to the wing. But if the model glides well it is more likely downthrust needed.