I love Gravity Falls! One of my favorite shows ever!
I started by photo-shopping a screen-shot of the closeup of the Meow-wow sweater to make it forward-facing, remove the shines and color adjust. I printed the design onto transfer paper which I then ironed onto some cheap white cotton muslin. I cut that out and hand stitched onto the sweater. I bought a spool of pink glitter ribbon which I used to make the "Meow Wow!" letters and hand stitched them onto the sweater. The neck was extra long so I folded it over once and pinned it down to hold the standing up shape.
The skirt I made from scratch and the fabric looks much more green in person but I've noticed it tends to photograph much more blue-ish teal. So I might remake it later. :P
I bought the wig from http://www.wigisfashion.com/ and the headband I found in a clearance bin at Michael's for $.99! lol