In model rocketry you must ensure your rocket is stable. A stable rocket will fly perfectly, while an unstable rocket will fly in an unpredictable pattern possibly ruining the rocket, damaging property or hurting someone. Stability is extremely important when building a rocket from scratch, and a good idea to know how stable your kits are also.
In flying, and designing, model rockets you must understand and know the Center of gravity (CG), also known as the center of mass, and the center of pressure (CP). The CG is where the rocket balances, motor loaded and recovery system installed, along its length. The CP is the average location of all the forces on the rocket from passing air. You always want the CG to be in front of the CP for a stable design. Adding nose weight, a payload, or lengthening to rocket will move the CG forward. Increasing the fin area will move the CP back. A typical rule of thumb is to have the CP 1 caliper (1 body tube diameter) behind the CG. If your CP gets ahead of your CG the rocket will tumble or turn and go in an undetermined direction. If the rocket is over-stable (CP a long way behind the CG) it will be more influenced by wind and cross-breezes when it leaves the launch rod. How do you find your CP? You can use software like RockSim ($125 program by Apogee Components) or OpenRocket (Free Open source). Choose a kit from its database or design your own rocket. If you don’t have the software or just don’t want to use it, there are other ways. The cardboard cutout method is a fairly effective low-tech approach – a cutout of the lateral (side-ways) outline of the rocket is made (lay your rocket on a piece of cardboard and draw around it with a marker) and the CG of this cutout (the point that you can balance the cut-out) is your estimate of the CP.