- Define your inputs (i.e., control surfaces on the fins, steerable motors, etc.)
- Define your output (roll position of the rocket)
- Find the differential equation that describes the roll in terms of the input.
- Linearize (note that if you're using fins then the linearized equation will be dependent on airspeed, and that if you're using steerable motors then the linearized equation will be dependent on thrust -- this will complicate your life, but get over the first hurdle first!)
- Extract the transfer function
State space is not a bad way to go, but if you're only concerned with roll control, and if you're confident that you're not going to be affecting the behavior in yaw or pitch, then this is a classic single-input, single-output system that can be handled with transfer functions.