My kids love Adventure Time. They decided they wanted to be Finn and Jake for Halloween. The cartoon is pretty new and you can't find any Adventure Time stuff in stores so I had to make their costumes from scratch. Here's what I came up with for my boy's Finn costume.
Step 1: Finn's backpack
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.