I talked a little about Autumn, but didn't spend a lot of time on this since by 4th grade the students are pretty educated on seasons already.
First, I had them draw the structure of the tree with a sharpie marker. I did the drawing with them, encouraging them to draw a large tree, and having them draw the trunk thicker then gradually get thinner until very thin at the ends of the tree. I also had them draw the tree a little to the side, with part of it going off of the page, instead of directly in the center, just to maybe making the composition a little more appealing.
It's also good to have them do a light coating of glue over the tissue as well as under, to make sure the pieces are glued well.
This is a very colorful project that looks very lovely in the classroom window as a group.