1. Open Titanium, hit Menu -> More -> Create Update.zip
2. You want Edify, User Application & Call it Titanium XXX.zip, then hit create
3. Place the main ROMXXXX.zip on the root of your SD Card.
4. Open ROM Manager, look at the top option, if the current recovery isn't ClockworkMod 3.0.1.4 (for Gen 1 handsets), hit Flash Clockwork Mod Recovery. If you get a prompt to chose your handset, it should hopefully be an easy choice. Let it do its thing.
5. Still in ROM Manager, press "Reboot into Recovery", after a 30 seconds or so, your phone will boot into clockwork.
6. You'll need to use the Volume Keys to navigate the different menus, and the power button to select, scroll down to Backup & Restore and take a backup of your current ROM (I cannot stress how important this is). It may take anything up to 10 minutes.
7. Once completed, head to the "Mounts and Storage" menu and from there, individually format the system, data & cache partitions. You'll need to confirm each one which can be annoying, but it will hopefully stop mistakes.
Don't format the boot partition. Don't reboot.
8. Head back into the main menu in clockwork and select Wipe Data/Factory Reset (Again, you'll need to confirm).
9. Once completed, select "Install Zip from SD Card"
10. Scroll down to the ROMXXXXX.zip and select, again confirm. This will install your chosen ROM on your device.
11. Head back to the main menu, then select Install Zip from SD Card and choose the titanium zip you created at the start of the process.
12. Head back to the main menu, select reboot now. 1st reboot should take 10 or so minutes.
13. Read
this to see about restoring your apps and data etc.