Brainstorming sessions generate a ton of ideas, but most are just fragments. Drawing an idea out on paper allows you to investigate an idea more fully, figuring out details and addressing potential pitfalls or challenges as you go.
Choose an idea that excited you from a previous brainstorm session. Make a quick list of all the questions you and others have about that idea, and the things that might keep your solution from working. Spend some time figuring out how you can refine your idea to work around or through those problems.
Reserve at least 10 minutes for drawing your idea. Using as much detail as possible, show your solution in action. It might help to use a storyboard or comic book format to show people interacting with your solution and/or show how it develops over time. You can choose to use words to explain details, or push yourself to represent your idea using only images.
Share your drawing, either with a partner or within the larger group. You can explain your idea, or for a bigger challenge, ask them to describe your idea just by looking at your picture. Have your partner(s) ask lots of questions and point out other possible problems.
Now re-draw your solution, using the feedback you got to make it even stronger and more detailed.