As root you edit the /opt/zimbra/conf/zmztozmig.conf with the correct values for your servers and the options for what you want to migrate.
Then as zimbra user you run /opt/zimbra/libexec/zmztozmig and it does it's magic. There is a good bit of information in the RFE here:
Bug 30163 – zimbra-zimbra migration via tar formatter
I'm stuck right now trying to figure out how to move mail filter rules since this tool doesn't do that for me.