You put in a 10.6 DVD, format the hard drive with disk utility, then install the OS. Simple.
Let me clarify a little more, stick your install disc, you know the one you got with your machine or whatever OS you have in there now disc in and restart the computer.
Hold down C after the bong which will boot into the CD, first thing you want to do when this boots up is go up to utilities, then disc utilities, then click on the harddrive, erase disc, your HDD will erase.
As soon as this ends, close the window and proceed with installing Leopard/Snow Leopard or whatever Mac OS you have on your computer, easy as pie.
however, if you need some visual instructions, this should help.
How to clean install Snow Leopard
