Well, I think with a question like this you have two main approaches that you can adopt. You can go through the novel and pick out key scenes that you feel are important and use them for your collage. One scene you might want to pick would be when Stacey and his siblings plan to make the bus for the white kids stop by digging a ditch and waiting until it fills up with water. Another might be when Cassie is forced to get off the path and into the road by Lillian Jean's father. Any of these moments would provide a clear visual description that you could use as the basis for drawings in your collage. Take the way the bus stops, for instance:
But instead of the graceful glide through the puddle that its occupants were expecting, the bus emitted a tremendous crack and careened drunkenly into our trap. For a moment it swayed and we held our breath, afraid that it would topple over. Then it sputtered a last murmuring protest and died, its left front wheel in our ditch, its right wheel in the gully, like a lopsided billy goat on its knees.
Picking out important scenes like these that make up the key elements of the plot would be a good place to start.
Your second option would be to consider how you could represent the key themes of the novel more metaphorically, through focusing on symbols and motifs. For example, an obvious theme of the novel is the way that racism impacts the lives of the Logans. Trying to work out how you could express this thematically through pictures would be a much more interesting and novel way of completing this assignment. Hope it helps and good luck!