WPMU Upgrade step-by-step

My copy of WPMU just informed me it was time to upgrade. Since I installed it last night, there'd be no great loss if I screw up, but I thought it'd be nice to get in the habit of best practice.

What's the correct step-by-step to make sure I update this correctly? I assume there's a MySQL database save of some sort, along with copying file trees, but some "for a moron" details would be most appreciated.