OK, first you build a frame on the frame you connect the steering to that the front wheels and the steering wheel. Next attach the motor to the frame and the rear axle. now connect the rear axle to the motor with a bicycle chain and then put the wheels on the back axle. Now attach a chair to the frame. and your done.
I dont know of any free plans, but if you were to go to a store and look at one you could probably figure out a way to make one yourself, they're pretty simple to make. the most dificult parts are connecting the motor to the rear axle, and the steering.