To do this, I taped a piece of clear contact paper to the sliding glass door, sticky side out. Then I gathered a basket of different materials to add to the collage--puffballs, ribbon and bows, sequins and jewels, etc. The kids had a blast sticking the different materials on. Little Bean even started arranging the ribbons in a pattern on his collage.
Once they were finished, I removed the contact paper from the window and applied another piece on top, to hold everything together. These look beautiful hanging in the window, or just framed on a wall.