Well you know I'm going to say "it's in the FAQ", but it is
But the basic steps are:
* do your Ti backup (done), and any other app/data backups you want. Ti covers everything, but there are solutions for stuff like call logs and messages which are simpler to use (e.g. MyBackup), so some of us use these in addition.
* make a backup from recovery (i.e. a "nandroid" backup). Reboot the phone, and if you have problems rebooting, go back into recovery and restore the backup you just made (doesn't happen to everyone, but if it does this will fix it. Only happens once).
* Download the ROM to your SD card.
* Go back into recovery. Do a factory reset - that's why you needed the app/data backups, because you just wiped the phone, but you have to do this or else the phone won't boot with the new ROM (or maybe it will, but there will be other odd problems).
* Then select "install zip from sd card", select the ROM zip and install it.
* Reboot the phone. First boot may take a while.
* Install Ti from the market, and restore your apps and data. Do not restore system settings, just "missing apps and data".
* Have fun setting up your new ROM.
The nandroid backup is in case anything goes wrong, or you forget to back up something vital - restore from that and you are back with you old ROM/apps/data.
The other thing is to decide whether you want to use a2sd+ - if you do, it's probably simplest to do the preparation (adding an extra partiton to the card) before flashing a new ROM. This is described in the root memory FAQ. Be aware that partitioning will wipe the SD card (including your backups), so you need to copy
everything to a computer before doing this.
Hope that helps. As I say, there's nothing here that isn't in the FAQ, all I've done is selected a few bits from memory, so do read and re-read the FAQs before doing any of this. But maybe this will help identify which bits of the FAQs you should be looking at.