Take your small car base piece and attach a 4 x 12 long piece to the back four raised studs of your base. This will form the bed and front cabin of your truck. The recessed area of your small car base is where the cabin of your vehicle will be and where you LEGO driver will sit. Put the steering wheel and the seat in the dropped-down area. Start by taking both your square windows and stacking them, one on top of the other. Then attach these in front of the steering wheel to complete your windshield. Your fenders, sometimes called a rounded mudguard, should be half circles attached to a flat piece or brick. Connect your fenders in front of the windshield, so the half circles point downward on either sides of the front of your truck.

Summary:
Create the frame of your truck. Add cabin features to the lowered section of your base. Create the windshield of your LEGO vehicle. Fasten fenders to the front of your car.