How do you get a rounded off nut off?

There are many approaches that can be used on a rounded nut or bolt. Many of them depend on the access you have to the nut. If you have easy access you can try using Vice grips, a pipe wrench, or a hammer and chisel/screwdriver. You may also consider applying heat to the nut or welding another nut to the existing nut if you have the proper equipment.

You should put some penetrating oil on the nut, penetrating oils such as liquid wrench help you break the nut loose.

Other tools designed specifically for removing rounded nuts include a nut splitter and bolt/nut extractors

(See the website at the related link for a discussion on removing a rounded nut with a hammer and screwdriver, cutting tool, penetrating oils and nut extractors.)