That's too general a question, rockstar. You need to say more about the application, voltages, current levels, etc. And whether you want to use a simple analog circuit, or a more sophisticated microcontroller (uC) based circuit to do the PWM. Is this for the RC vehicle application that you mentioned in another post? If so, what does the controller look like in the standard circuit before you started modifying it?
there is no standard circuit to start from. i was hoping something as simple as a 555 ic and it'll be controlling the H bridge mentioned in the other post. for simplicity i just built a standard h bridge for now out of uprated parts, the H-bridge and the P.w.m. will be a seperate chip from the radio controls, for the sake of prototyping (sorry about the vague question)