I started by buying a pattern for a baby fleece suit and then I looked up some pictures of Gizmo for reference.
I noticed that Gizmo has white fur around his wrist, up the front in a strip, and on his legs.
I laid out the pattern I had bought and laid tissue paper over it. I traced out the original pattern and then I drew in the panels I needed to be brown. I cut the new panels out of the tissue paper making sure to include a 5/8" seam allowance.
After this I pined the pattern pieces to the fleece and cut the pieces out.