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.