Normally the instructions will have you build the airframe. Cover it and straighten any minor warps in the process. Then install the gear that has to go in a certain place. Engine, fuel tank, tubing, prop, spinner, landing gear. On a trainer usually everything has a specific place OR you can slide the servo tray forward or back to balance.. If not mark the CG on the wings and place the servos and battery pack temporarily until the plane balances on the CG. Mark those servo locations and build your servo tray so you can install the servos to balance.