A 60-grit belt for a belt sander, if you have a hand held, chuck it in a vice upside down, rig a vacuum to catch most of the dust and wear a respirator, and a fan blowing away from your face.

This is the fastest way and will leave a smooth surface, but will trash the belt. Let the belt do the work and use light pressure. Use a straight edge and measure the thickness to create a flat, surface parallel to the other side.

Or 60 grit Silicone Carbide, a steel cookie sheet from the dollar store and a flat piece of cement, (garage floor) will get you flat in 20 minutes or less. Mark a grid with a sharpie and use your body weight. GOTGRIT.com will have all the grits from 60-500, ($15) then go to Wet and Dry once flat.

Stay on the 60 grit, adding a half teaspoon at a time with a squirt of water. The cookie sheet will contain the mess.

Do not use a diamond plate, you will trash it.