Just notes for posterity: I am installing Cyanogenmod on my HTC Tattoo Android phone.
There are instructions here which are perfectly understandable if you're comfortable with a command-line. But the instructions are incomplete. As has been discussed here I needed to install "tattoo-hack.ko" in order to get the procedure to complete (at the flash_image step I got "illegal instruction" error). Someone on the wiki argues that you don't need that instruction if you're upgrading from stock Tattoo - but I'm upgrading from stock Tattoo and I needed it.
In the end, as advised in the forum thread linked above I followed bits of these instructions and used tattoo-hack.ko as well as the different versions of "su" and "flash_image" linked from that forum thread, NOT the versions in the wiki.
Also, there's an instruction in the wiki that says "reboot into recovery mode". It would have been nice if it had spelt that out for me. The command to run is
adb reboot recovery