I found this idea on Pinterest (http://racksandmooby.blogspot.com/2010/10/jessie-toy-story-toddler-costume.html), and just adapted it using things I had on hand.
The pants are a pair of my daughter's jeans with cow print felt safety pinned to it. (The felt sheets came from Joann Fabrics. I just cut the felt to fit her jeans.) You could use fabric for this as well, but the felt was very easy.
The belt is a piece of black felt. I hot glued a circle cut from a red glitter foam sheet. (I think I bought this at Joann Fabric too.) I safety pinned the belt to her pants.
The cuffs are pieces of yellow felt. I placed velcro sticky circles on the felt so they could wrap around her arm and stay in place. I hot glued red glitter ribbon to the felt for the accents. I also hot glued some white ribbon to the cuffs.
The top yellow is cut from a piece of sticky-back yellow felt. (These sheets came from Meijer.) I also hot glued red glitter ribbon to that felt and then just stuck it to the shirt.
We had a red cowboy hat, so I just taped a piece of white ribbon around the hat.