When I cleaned my off I had my engine on a stand so this was a little easier to do. Rotated it so that the side I was cleaning faced down and slightly to the side. The bores on either side of the piston I was cleaning where tapped over so that no dirt got in them, also keeping the holes for the head bolts covered. Rotate the positions till the one you want to clean is at TDC and used a wire wheel to clean it.
The bore was lubricated with a thin coating of oil since the engine had been out for an extended period of time.
I took a wire wheel and went over the large portions of the piston till it was clean
The small dishes in the piston easily cleaned with a brush. Getting all the little bit out if there can be a pain but it completes the job.
When all the work was done the finished piston was rotated back down the bore the oil caught most of the dust.
I cleaned the oil away from there and the job was done. Also make sure to wipe down the piston top for left over dust.
A nice repeat process to go on from here and of course a double triple check at the end just remember how little it takes to kill a bearing.
... for an engine in a car i would use the same process but be even more careful about dust falling down into the ring land while cleaning. best of luck.