Once the computer boots into the Leopard installation, keep following the menus until an "option" button appears at the bottom left. Click it. Then, choose "Erase and Install." From there, you will get a clean install of 10.5. Keep in mind that you will have to reinstall everything again (even iLife), so make backups and be ready.
Startup the MacBook in Tiger, put the Leopard disc in and it will launch a program asking if you want to install Leopard. Select Yes, then click the Restart button it presents you with, that's all there is to it. It will reboot the Mac and launch the Leopard install disc automatically. Then you just go through the install routine as you would with any OS.
This doesn't cover things like de-authorizing your iTunes account, backing up data and so forth. So, you'll want to make sure you've done those things first.
If you want to be really safe, you could even completely recreate the partition map, erase the drive, or even go one step further and zero out all data before proceeding with the installation. This can be done via Disk utility on the Leopard installation DVD.