A while ago I did a tutorial on installing Mono 1.2.6 on Ubuntu. As it turns out, installing Mono 1.9.1 follows the exact same process. You can thus use the 1.2.6 tutorial as is, just replacing the version numbers in the files you need to download with the corresponding 1.9.1 versions. The only notable exception is mod_mono which is only version 1.9. All full list of exact file locations is available from here: http://ftp.novell.com/pub/mono/sources-stable/
I haven't yet tested the installation procedures on Ubuntu 8.04, but I assume they'd be pretty much the same. If I'm wrong in that assumption, please let me know.