Before considering a dolly, get the car's manual out and read the section on "recreational towing" or similar. Not all automatics can be dolly towed.
In answer to your question, I used to dolly tow a car with a low front end. I made a couple of 2 foot lengths of 2x10. I laid them on the ramp about 1 foot up, extending out behind the dolly about 1 foot. That raised the front end of the car enough to get onto or off of the dolly. I had 2 screws in the underside of the end that went on the dolly ramp that went into holes in the ramp. That kept the 2x10's from sliding farther up, or off of, the ramp.