It's normally an Intelligence check
As Miniman's answer suggests, both pulling together a disguise and forging a document are listed under the Other Intelligence Checks section of the Ability Checks guide in the PHB, so that would be the default.
This might, for example, be a History, Nature or just a generic Intelligence check (with a DM-supplied DC), representing the need for some kind of special knowledge (remembering exactly what a certain race is supposed to look like, or knowing all the right names to include in your document, or remembering what someone's handwriting looks like, etc.) to create the disguise or forgery effectively.
At DM discretion, other checks might apply
In some cases, I could see a DM requiring a Dexterity check (instead of or in addition to the Intelligence check) to see if you are physically capable of the fine control required to copy someone's handwriting convincingly, apply detailed makeup, etc.
After creation, there may be additional checks required to get away with it
Under the description of Deception checks, one of the examples is pass yourself off in disguise. Thus once you've created the disguise using the kit (and most likely an Intelligence check) you may still need a Deception check to pull it off.