A step-by-step guide would include the fact that you have a number of options once you have put in the Tiger install disc.
I'll copy and past from Apple site:
if you click Options on the "Select a Destination" window, an installation sheet will appear asking you to choose between Install Mac OS X, Archive and Install, or Erase and Install. Select a method, then click OK to continue.
Install Mac OS X choice
This installs Mac OS X for the first time on a disk that does not have Mac OS X currently installed.
Archive and Install choice
This option moves existing system files to a folder named Previous System, and then installs a new copy of Mac OS X. This option is available only for volumes with Mac OS X already installed. Usually, if you select this method, you will want to also select (check) "Preserve Users and Network Settings" which imports existing user accounts, home folders, and your network settings into the new system.
Erase and Install
If you want to erase your disk before installing Mac OS X Leopard, select this option then click OK. Important: This option erases the entire disk, so make sure you've backed up your important files first.