Run virt-manager
Click "New"
Enter any name you please
Select "Local Install Media"
Select "Use Iso Image" -> Browse -> Browse Local
Select the Ubuntu Server 12.04 ISO you've downloaded earlier
OS type: "Linux" Version "Ubuntu 12.04 LTS"
Select at least 512 MB of RAM
Select atleast 1 CPU
Check "Enable storage for this virtual machine"
Check "Create a disk image on the computer's hard drive"
Make sure you select at LEAST 4GB, if you have room consider choosing 15GB
Check "Allocate entire disk now"
Click "Advanced options"
Select the default network device (will be changed later), alternatively "Host device vnet0 (empty bridge)" to use bridge just set up
Select "Set a fixed MAC address"
Run randommac.sh and enter its output into the MAC address textbox:
Virt type: kvm
Architecture: Make it match the iso (and hopefully your host computer)