ACNH Design Pattern Editor , An amazing app for Windows and Mac, which allows you to easily create and manage patterns. layers, image conversion, QR code support, and much more is in this ! I highly recommend this!
acpatterns.com , a website that allows you to create, share, and export ACNH designs. allows image conversion as well.
NHSE , a save editor for ACNH that can edit saves which can be re-injected using a tool such as JKSV. Download is to the right under "About" This is the safest way of editing a save. I do not recommend using methods such as "live hacking" at all.
Villager and Villager House files for NHSE , Unlike the last game, villagers and their houses are actually separate from each other. To properly change out a villager in NHSE you need to make sure you have the correct files. The link given shows you how to do this properly.