If all u want to do is take off the gloss, then u dun need a buffer. Buffers are used to shine up the finish. I would recommend taking everything off. All ur electronics, neck, pickups, screws.
Hand rub with the 00000 steel wool (Get it from Home-fix, $25 a pack), don't mix it up with normal steel wool, this is very fine stuff. Be careful when rubbing down corners (right angles), as they are prone to "sand through". Basically how u get a matt or satin finish is to introduce very fine scratches on the finish to diffuse light, what you want to do is to get full and relatively even coverage of scratches.
But make sure you're serious bout the change, cos once u satin the finish, its very very hard to bring it back to a full gloss. Think through impact on resale value, etc.