Want to install Tiger
 

Currently using 10.3.9
Don't want to upgrade, instead i want to install Tiger.
I'm already backed up to an external...

Please confirm the following steps:
1. Make Final backup.
2. Change name of external from xxxxx to Macintosh HD
2. Reboot from external with option key at startup
3. Erase Internal
4. Install Tiger
5. then?

Can I ask why you don't want to upgrade to Tiger? You can always "clean install" after you upgrade, if you find it doesn't work as expected, but you can't go "back" to an upgrade after a clean install...