Making your model rocket fly correctly.

Oh the nervous shaking of your hands while you hold that electronic launch controller. All those hours of cutting, gluing and painting have come down to this moment. 5-4-3-2-1 Push the button! Your rocket leaps off the guide launch rail perfectly, and then turns 90 degrees and flies of into the ground some distance away. This will be all too familiar to anyone who has build and flown a scratch build rocket. There is some serious science behind making these things fly right. There are things called CP and CG and Thrust to weight ratio, forward-swept fins, trapezoidal fins, ogive nose cones and many other factors that influence the flight of a model rocket. I will try and explain some of the basic rules you will need to consider when designing and building your very own rockets. The first, and possibly, most important relationship in rocket design Is CG/CP or “Center of Gravity”/”Center of Pressure”. The Center of Gravity is the point on a rocket where it balances according to the distribution of weight of the components.
Fig 1.0: Center of Gravity.

Center of Pressure is very similar to center of gravity, except, the forces are aerodynamic instead of weight (gravity). The center of pressure is best calculated mathematically, however, it is possible to estimate the location of CP but using a cardboard “cutout” of your rocket. Draw the outline of the rocket on a piece of stiff cardboard and the location of the center of gravity of this cutout is approximately the center of pressure.
Once you know where CP is, all you need to do, is make sure that this point is at least the width of the body tube closer to the fins than the center of gravity.
Fig 1.0: Center of Gravity/ Center of Pressure.
As long as you keep this condition right, and your motor is powerful enough, your rocket will fly high and straight.

more model rocket info here