I am not a web developer, so please let me know if you are and there is
anything here that you would like to help with, or that I can easily
fix.
How do I use this?
First, you probably want to the images you want to use yourself ahead of time using something like The GIMP or Photoshop. This can optionally resize images for you, but dedicated image editors will give much better quality, and let you specify your scaling algorithm (some images will look better resized with interpolation, for instance, and others will be better with a nearest-neighbor/no interpolation). Regular design sizes are 32x32. Also make sure, if you are using Pro designs, that you are using images without any transparent colors, as transparent colors will not work with Pro designs.
Select one or more palette images to generate the palette. You may select more than one here to ensure you can maintain a constant palette for Pro designs.
Select the image you want to load.
The design should now be generated in the table below.
Go into the palette color editor and edit each individual palette color. The Hue, Saturation, and Value sliders are numbered from 1 from the left.
Fill out the pixels. If you are doing a pro palette, make sure you have used all the images you want to use to generate the palette, and load your next image. Feel free to tweak the ditherer, but do not touch the optimizer setting, because the palette may not be changed for different parts of a Pro design.
Why not just use the QR code generating ones? Isn't this tedious?
This is extraordinarily tedious. This tool exists because Animal
Crossing: New Horizons has a fully-customizable palette, and Animal
Crossing: New Leaf did not. Unfortunately, the QR codes only support the
older palettes with limited colors, so the only way to get the full color
fidelity that New Horizons makes possible is to tweak the palette and put
pixels in individually. At least unless Nintendo updates Animal Crossing:
New Horizons to support any method of sharing full-fidelity patterns without
going through their servers.