Short form
improv, also called "theater games" or "improv games," is a relatively easy form of theater to perform and produce, especially if you have a great group of funny people that like to make each other laugh.
Instructions
1. Understand the difference. The key to short form is finding the joke, finding it fast and moving on to the next game. This is different than other types of improv, such as long form. In short form, the scenes are almost always very short and focused completely on finding the most expedient way to the kernel of funny. Long form scenes tend to focus more on developing a relationship that the humor grows out of. Because of this, a short form performer must develop lightning quick joke finding skills, which come with practice of the different games that short form groups perform. It is also key that the group dynamic be tight, but fun.
2. Learn the games. There are a myriad of games that audiences love watching. A few good ones to start with are "World's Worst," "Story Styles," "The Props Game" and the all time drama school favorite, "Freeze Tag."
3. Know play "World's Worst." All participants step to the front in a line and receive a suggestion of an occupation from the audience. They then take turns giving examples of what the "World's Worst _____" might do. For example, World's Worst President: "Thanks for the tour of the Oval Office... say, what does this red button do?"
4. Play "Story Styles." Participants stand in a line and the first starts telling a well known story, like "Sleeping Beauty," but as the story is passed down the line, each participant tells the story in a different style. One might be Film Noir, the next might be Romance Novel, while the next might be Western.
5. Grab some stuff to play "The Props Game." It's a very simple, but potentially hilarious, game where two sets of two people are each given an interesting looking object and they trade back and forth creating one line situations using their respective props as something that they clearly aren't. For instance, if a group is given a banana, someone might "answer" it, making it a telephone.
6. Add "Freeze Tag" into your line up. It's fun because the possibilities for play are limitless. Two people start an improvised scene (preferably one with a lot of physicality). At a moment of physical awkwardness, a third participant yells "freeze" and replaces one of the two actors on stage in the exact same position. When the scene starts up again, however, it must be a new scenario working into account the awkward positioning of the two actors. This game is often used as a warm up because of its demand for the actors to think on their feet.
7. Develop a dynamic. Rent a room once or twice a week and get the group together to practice all of the games for a couple hours. Have a director or coach present to give an objective opinion about when you're really hitting the jokes and when you're getting off the mark. Once you've practiced by yourselves, you must get in front of an audience. The more your group performs for a group of paying audience members, the better they'll get. If something doesn't work in front of a group of strangers, you'll know immediately. Your group will benefit from it and your short form will improve because of it.