First, I would decide on what type of upgrade you want to do. Go over to the poll thread about what kind of upgrade people are going to do.
Second, I would make sure you have a back up of all your important docs and music and photos and other such things. Apps can be re-installed if they are lost.
Third, Insert OS X Tiger CD and restart your computer. after you hear the Chime hold down the "C" Key and it will boot from the CD.
Fourth, after booting from the CD it will go through some Liscense stuff. After that you will see the install option and it will allow you to choose the disk you wish to start the install on. Unless you are doing just a straight upgrade you will need to click on the options button and choose your install click the next button. Then you will come to the install screen and you cna choose to do an easy install which will install everything or you can choose the options button then choose what you want installed. I usually uncheck the Language translations unless you need them. Also I usually uncheck all the printers except for the brand that I'm using. After that click the install button and wait.
You shouldn't have to wait long since the install is amazingly fast. I think I waited around 15-20 mins for the install to complete.
After install it will ask you to reboot. Reboot and enjoy Tiger. I would go into the utilies folder and run disk Utilities and repair permissions first. After that you should be all set.
(disclaimer: The options or text that I stated here are as accurate as I can remember. They may or will be different.)