1: Get all your data off first obviously, disconnect all other drives and media.
2: Stick the SL disk in the computer and reboot holding the c key down, you'll boot off the disk
3: First screen is the language, second screen is the installer, but wait, look under the Utilities menu for Disk Utility, launch that.
4: On the left you'll see a bunch of things, your hard drive makers name should be at the top left, select that.
5: Next click the Erase tab thing, click Security Erase > Zero All Data (leave the formatting HFS+ journaled) and then click Erase... come back in a hour or two. If you need more security pick the 7x erase, it will take more hours and really scrub the drive to military standards.
6: Once that is completed, quit and you back in the installer, install OS X SL onto the hard drive.
When it's finished you can time it when it starts to reboot and hold the power key down to shut it down hard.
When the next user goes to boot, they get the Welcome Screen and initial setup, so it's all ready.
All the OS X bundled programs are included on the SL upgrade disk except iLife, that's on the grey disks that came with your computer and to get that back you first have to install OS X from those disks first, then setup a initial account then use the SL disks to upgrade.
I'm assuming you don't have the grey disks, so that's why your installing from the white SL upgrade disks. (which really has the whole OS, not a upgrade)
If you do want iLife installed, then you need to install from the grey disks first, the proceedure is just about the same on 10.5, hold c and boot, use Disk Utility to Erase the drive, install 10.5, reboot, go through the initial setup, then insert the 10.6 disks (no c boot needed) and simply double click on the updater installer and it will handle the upgrade to 10.6
Usually the next person is going to wipe everything off anyway, but then again they may not. You'll have to pick some sort of nick name to enter into the user fields, and skip the registration process.
The important thing is to use the Security Option> Zero all Data (or 7 pass erase), this scrubs the entire drive of all YOUR DATA, so it can't be recovered using certain software.