Attach a sanding disk of about 250-grit to your grinder. Test the grinder on a small piece of aluminum, to see the results before working on a larger area. Lubricate the aluminum with a kerosene-based polishing oil. Firmly secure the test piece with clamps, to prevent it from shifting as you grind it.
If the wheel looks as if it's getting gummed up from melting aluminum, add more lubricant and try again -- grinding at a slower speed. Once you are satisfied with the results, clamp the larger piece into place and lubricate the surface.
Move to a higher grit, once you have finished grinding with 250-grit. With every higher grit you use, remove all the marks from the previous grit. Achieve a nice, polished surface by finishing with 500-grit wet sandpaper. Mix some soap with the water and grind the surface, to remove any remaining marks.
Etch the aluminum using the grinder with grinding wheel attachments, as opposed to sanding disks. Use lubricant and avoid grinding at high speed, to keep the metal cool.
Grind hard-to-reach areas with a hand-held carver, using the same grit as you use for the angle-grinder.
Experiment with different types of grinding wheel attachments to see the effects. The type of grinding you desire will determine the attachments you need.