Its an extremely easy process.
I'd recommend you use Ubuntu Server 10.04 though, but upon install make sure nothing extra gets installed.
Once its installed and setup with a static IP
Split DNS - Zimbra :: Wiki
Set up dnsmasq
Then download and untar the Ubuntu ZCS tar.gz file, then run the the install.sh as usual and you should be good to go.
I'd post my self made guide I have but I'm at work at the moment.
If your looking for a guide to guide you through exporting Exchange to Zimbra, good luck as that can be a pain.