There is a certain order to trimming a plane and that can vary a little from pilot to pilot. Most pilots will concentrate on the elevator trimming first to keep the plane flying a level path. This is especially important if the plane wants to dive, then elevator trimming becomes the primary important trim for the pilot. In most cases you will be trimming the ailerons along with the elevator to keep the wings level. Once both these control surfaces are trimmed to a comfortable standard you can begin trimming out the rudder.
Now some pilots will not even bother going past the elevator/aileron trimming step, and never bother to trim the rudder at all. But it is very important to trim out the rudder for nice straight and level flight. Rudder trimming starts by performing figure eights in the sky and watching to see how the plane reacts. If you are making a left turn and the plane wants to climb during that turn you will need to add left rudder. If the plane wants to dive then you will have to add right rudder. Now when you are turning the plane to the right if the plane wants to climb you will have to add in right rudder, likewise if it tends to dive trim in left rudder. Continue flying the figure-8’s until your plane performs nice smooth turns through the right and left banking.
Keep in mind that throughout this whole process you will most likely have to add in small increments of both elevator and aileron trim. Generally when you are trimming out one control surface it will affect the others. So just take your time and realize that trimming out a plane can be an ongoing project. This can be especially true if your first flight is on somewhat of a windy day. Always take wind into consideration when trimming your plane that is not to say you shouldn’t trim out your airplane on a windy day. But just understand that it may need some slight tweaks the next time you fly on a calm day.
Try trimming out your planes following these suggestions and I am sure that you will find your plane flying a little better. A properly trimmed out plane is always fun to fly and is always worth the time invested to get it right. Enjoy.