Balancing your aircraft (setting the center of gravity) is extremely important if you want your model to fly properly. Most models are fairly simple to balance. Here is how to do it. 

BALANCING YOUR AIRCRAFT:

Measure from the leading edge of the wing back the distance indicated on the plans, around 50mm out from the fuselage on each side, mark with a black texta.

With the battery in place and the plane in a ready to fly configuration, balance by placing 1 finger on each texta mark. If the nose starts to sag down very slightly with your fingers on the marks it will be within the acceptable CG range.

 If it balances with the tail down and your fingers are on the texta marks then the model is too tail heavy and you need to move your battery forward or add nose weight until the model balances very slightly nose down when your fingers are on the COG marks.

If you do some test balancing during installation of the radio equipment and battery, you can usually move things around and not need to add dead weight to the model.

The radio gear doesn't always have to be placed exactly as shown in the manual. Moving the servos forward a little, or re-positioning the receiver can make a significant difference in the aircraft's balance point.

When all else fails, add lead weights to achieve proper balance. The further the weight is from the CG... the more effect it has, so placing it as far forward or back as possible helps reduce how much you have to use. (Lighter aircraft fly much better! J)

After the fore and aft COG has been correctly set you can further check the balance laterally, IE from wingtip to wingtip.

Hold the aircraft by the spinner and attach a string to the top of the rudder fin. If the plane wants to rotate upside down, turn it over and do the test upside down.

  Whichever wing drops is the heavy side. You will need to add weight to the light wing. You can drill holes into the wingtip ends and fit wood screws in the wing tips to balance the model laterally, this is an easy way to obtain the correct balance.

The more accurately balanced the aircraft is, the better it will fly, the less trim it will need (subject to how square you built it!) and the easier it will be to keep in straight and level flight.