IF you don't have the disk, then you are a little SOL.
However, I will give you the instructions anyway, and you can fill in the disk part later.
1: Get external hard drive equal to or larger than your laptop's hard drive (this can also be done with a partition on said external)
2: Run Time Machine, backing up your data to the external
3: Insert Snow Leopard Disk
4: Run Disk Utility from the disk, and erase the laptop drive
5: Install Snow Leopard (I would advise clicking the customize/options button on the screen where you choose the drive it is going to, and deselect printer drivers and languages and whatnot, saves space and time)
6: Once it is done installing, it will ask if you want to import information, select time machine, make sure the external is hooked up, and go hog wild.
7: Do some laundry, go for a run, learn a second/third/eighteenth language as this may take a while.
Shazam.
Another alternative is to replace 'Time Machine' with 'CarbonCopyCloner', and replace 'Restore from Time Machine Backup' with 'Another drive on this computer'.