This activity is pretty simple to explain. First, I draw two stick figure outlines on the board in one color of chalk (the same as on the page - the head and the "body"). Then, in another color of chalk, I draw in the faces, limbs, and any other necessary objects or scenery. For example, I draw the two people with tennis rackets in their arms with one of them swinging. I ask the class "What are they doing?" and wait for a student to answer "They are playing tennis." It can be good to recite this with the class to make sure they're aware of the particular grammar form that you want them to practice.
I usually do three or four examples, possibly inviting the JTE to try one as well. I use the same base stick figures in one color of chalk and then draw in everything else with a different color to reinforce that this is how I want the students to draw their stick figures. Usually my examples are something like playing baseball, eating some kind of food, and swimming. If you're not artistically inclined, that's okay - it's probably better that the students see that you don't have to be a great artist to come up with something simple and funny.
Next I pass out the worksheets to all of the students in the class and ask them to draw their own scene. If you think the students will take too much time as they draw and erase their drawings a zillion times, you can require that they draw the pictures in pen instead of pencil. Next they'll need to show the picture to their classmates and ask "What are they doing?" Their classmates should write out what they think the picture represents. You can have them get up and ask classmates individually, or have students pass their papers around the class in some kind of organized fashion and then get their papers back once 8 people have written their opinion.