1: Most common way; When smoking a joint or a blunt, one person turns it backwards so the lit part is in their mouth (usually between their teeth). Another person puts their mouth on the other end (the end your mouth would usually go on) and sucks while the other person blows into it.
2: Have both people make a tight tunnel with their hands. One person takes a hit of marijuana and blows into one end of the tunnel while the other person sucks on their end of the tunnel.
3: One person takes a hit of marijuana and holds the smoke in their mouth. They press their lips onto another person and open their mouth while the other person sucks the smoke in.
"Does she want to shotgun weed?"