You don't have to do anything . . . it's automatic if you have the bandwidth (yes), have pretty good uptime ( a few hours a day is enough, I think), and can accept incoming connections.

See the sample bug report in the options, and scroll through it to see if you can find the line about incoming connections. If it says TRUE and you didn't disable ultrapeer capability in the options, you'll become an Ultrapeer when needed.

Cheers