For the backpack, find some light and dark green fabric. Felt holds it's shape best. You could also use fleece or cotton.
Cut 2 light green fabric rectangles 12 inches wide by 6 inches long. Cut 2 dark green rectangles 12 inches wide by 10 inches long. Flod these pieces and round the edges.
For the edge of the backpack: Cut one dark green strip 3 inches wide by approximately 29 inches long. Cut one light green strip 3 inches wide by approximately 20 inches long.
For the straps: Cut 2 dark green strip 5 inches wide by approximately 12 inches long (10 inches long if the backpack is for a child). Cut 2 light green strip 5 inches wide by approximately 10 inches long (8 inches long if the backpack is for a child).
Other things you'll need : One 16 inch zipper and one black button and some polyfil stuffing.