I thought about saying don't bother. But most people get on their high horse and presume its easy. Or think they can do it by hand with some sandpaper and 'elbow grease'.
If you're of the mind set that you can attack your cars paintwork with wet and dry paper without ever doing it on anything else first, you're either an idiot, or you cant have a very expensive car....
Sorry to sound harsh, if that comes across like it.
However, if you are looking to do it properly, do the relevant research, buy the correct products/equipment and then practice on a scrap panel. That link I posted gives you an idea of the stages to look for, i.e. fill the scratch, let it dry (for a few days) attack with a fine wet/dry paper, then machine polish.
(edit - just read you are looking to practice. good luck mate)