Like a handjob, but with feet. The most common methods are jacking the guy off between the soles, or between the toes of both feet. Also, the penis could slide between the big and second toe, while the other foot might be poking the scrotum, or perhaps having its toes sucked by the receiver of the footjob. There is endless room for innovation.
"She was sitting between my legs giving me a footjob, and then she leaned forward and blew me. It was incredible."