on ur left joycon theres a lil button underneath the 4 arrow buttons. that's used for capturing screenshots (press once) and the last ~30 seconds of gameplay (press and hold). i havent done this myself, but i assume u can enter the passport app on ur nook phone, take a screenshot, and then mess around with cropping and stuff on a computer/phone, and then make it ur signature. people may also just take a screenshot of their passport in the passport app on the acnh portion of the nintendo switch phone app (im so sorry if thats confusing)