Essentially, the character would set various attributes on the disguise kit. Thinks like disguise name, disguise description, disguise gender, disguise short description and disguise nakeds. This can kind of be RPed as them making a mask, wigs, straps to constrict their bodies, wearables that bulk them up a little, Fake beards, moles, colored contacts and all kinds of things that, when donned, make them look like someone else.
Once it is all set they can don the disguise. When they do so their name, gender, short desc, desc, and nakeds are all replaced by those of the disguise. I would not allow clothing to be included in any this. If you were only wearing the disguise then you would appear naked.
It would depend on your relevant abilities and the relevant abilities of observes, when determining how much, if any, of the disguise descriptions are shown and how much of your own are shown. Maybe even a blatant "This person/feature appears to be fake." or something might pop up.
Not too sure on the details and there are a lot of them that would need to be worked out. Can someone wear a disguise made by another character? Can a disguise kit be re-worded with new descriptions or is one kit good for one disguise like with tailored clothing? Should a disguise be reusable or should it become trashed upon removal like Genetek jumpsuits? Should a disguise degrade over time?
Not even sure it's a good idea and I'm guessing it'd be a lot of work. Still wanted to throw the idea out there!