There are many versions, but a simple one has a circle in the center with 4 circles around it which are connected to the center circle with lines. (You could even fold a piece of paper into fourths and draw a square in the middle of the page. Trace over the fold lines, stopping at the center square.)
In the center, write what is being described: my bedroom. In each of the other circles, write something about the bedroom: my bed, my toy chest, the closet, my dresser. Now, you have a topic (bedroom) and 4 ideas you can use to write 4 sentences about the bedroom.
My curriculum doesn't do this until 2nd grade. (I teach a combined class of 1st/2nd graders and will start the Writer's Workshop described on this site during the first week of school, but it will be a simplified version at first!)
