The best thing to do is format the hard drive – completely erase it – and do an install of 10.6. That will give you the best results of a new and clean system, rather than an upgrade to 10.5 and then to 10.6. Formatting the drive will erase everything, so you will need to save files you want to keep.
When you’re ready, put the 10.6 Snow Leopard DVD in the drive and restart. (You need a retail DVD of Snow Leopard; a OEM disk that came with a newer MacBook won’t work, and is against the software license, anyway. You need a full version of Snow Leopard, such as these at Amazon.com Mac OS X version 10.6.3 Snow Leopard: Software or Best Buy Mac OS X v10.6.3 Snow Leopard – Mac or at Apple Mac OS X 10.6 Snow Leopard – Apple Store.
Push the DVD back in (if needed) and hold down the C key to start up from the DVD. It will take a few minutes to start, but you can let go of the C key after the DVD drive starts bumping along and you see the Apple logo.
Select English for the main language, and then you’ll see a screen that says “Preparing installation…” After that, you’ll see a bigger box “Install OS X.” What you want to do here is go to the utilities menu above and select Disk Utility. This will let you erase the hard drive.
After Disk Utility starts up, select your hard drive in the left hand pane. There will be two listed, one under the other, as well as the optical drive, so select the top most hard drive. Go to the Erase Tab and you’ll see a drop down for disk format. “Mac OS Extended Journaled” should be selected, but if it isn’t, select it. And then erase. Remember, everything will be gone, so if you have second thoughts about saving files, quit Disk Utility and the installer and go back and save them.
Once the disk is erased, quite Disk Utility and you’ll be back at the Installer where you can continue to install 10.6. Right after the install completes and you register, run Software Update to get the latest updates to 10.6.