Lay the loading ramp on the ground with the top of it facing the floor. Put the steel ramp on grass or some other forgiving surface.
Use the hammer to bang on the back of the ramp, forcing dents to well up on the front face of the ramp. Hit the ramp consistently throughout to cover the entire ramp with a bump every few inches.
Turn over the ramp and use steel sandpaper to score the bumps to improve their grittiness. You can also use a flathead screwdriver to hammer gashes into the face of the steel to dramatically improve grip.