Weight and Balance Belong in Pre-Flight
For RTF kits (that's Ready To Fly), you don't need to worry much about the weight and balance when you first buy the plane. However, you should always check the balance before each ﬂight. If the airplane is not balanced, it will likely crash. Planes, whether model or full size passenger jets, all have a center of gravity which must be within certain limits for the aircraft to fly successfully.
First, Check The Weather
This has a direct impact on the plane's balance. As a general rule, the center of gravity should be about one-third of the way back from the front edge of the wing (and two-thirds of the “wing-chord” forward from the trailing edge). To test, place the tips of your index ﬁngers under the wing tip, about one-third of the way back from the leading edge.
Then, very carefully, lift the model airplane up, balancing it on your ﬁngers. If the balance is good, the plane will be level, with the nose pointing horizontal, or maybe just a bit downward. If the plane's tail is pointing downward, then you have a balance problem and should not ﬂy the plane until it is ﬁxed.
Before adjusting it, think of what might have caused it. If you tested your model before leaving home and it was OK but now it isn't then what might be the cause? A loose screw or piece of material which has moved around during transit can be enough to affect the CG. If you now adjust it back to balance by adding another weight then you may have left a loose item inside your model which will surely come back to haunt you when it moves again during a flying maneuver.
More Model Airplane Secrets
If you are sure you have nothing lose and you do need to adjust the CG, add weight to the nose, something like ﬁshing shots, plasticine, or even modeling clay. Add just a little at a time, checking the center of gravity after each addition. (Or, you can move the engine more toward the front.) If you do not want to lose control and maybe crash and ruin your plane, this is a crucial step.
I am half way through the ebook, "Model Airplane Secrets" and can hardly wait to get my plane built.