Tee hee... he had to ask... Good job I am a fully qualified painter and decorator with spraying skills!
If its for the case (well, i shall use that as the example) sand down the paint using a coarse grade sanding material, using a sanding block to make sure it is smooth, and work your way down to lower grades, but DO NOT get rid of the paint, you DO NOT want to go to the metal... Then if you want, paint two THIN coats of an undercoat of some kind, a watered down (well, white spirited for oil paints) paint, that you will use as the final coat.
Then sand inbetween coats, but just a little bit, coz you will see a bit of a shine, get rid of that, but DO NOT go through the coat of paint you have just put on, then add very thin coats of paint, working slightly thicker each time, sanding between coats... then just before you add the final coat, dont bother snading, but always make sure you have got rid of the sanding dust, and give the surface a careful clean after getting riud of this dust...
And then you should have a very good looking paint job!