The first step to dressing kitties is learning how to cut stuff out for them to wear
then copy and paste another layer on top of the one you are using
"turn off" the bottom layer by clicking on the "eye" which you see on the right in the layer manager
just click the magic eraser in the white part
if it takes away too much change the setting where it says 10
or as a transparent gif if it is finished the way you want it