You will need the following mods: Litematica + MaLiLib. Mod Menu is also fairly helpful to get to the settings.
<aside> 🚨 NOTE: Any auto-building and block placement speed features, if present in any mod, are not allowed. This includes the use of Litematica EasyPlace.
</aside>
Firstly select an image you want to make a map art. Selecting a square shaped image helps but is not a requirement. Finding cartoony images can also translate better than realistic.
Go to Rebane and click on pick image:
Set Presets on the top left to Carpets, change Staircasing to Off (2D) and Add blocks under to No blocks. It can also be helpful to change Block to add to tinted_glass to help align later. You can use Map size to change the size of the map in game (1x1 is one tile, 2x3 would be 2 tall and 3 wide, totaling 6 maps to make).
From here you can adjust how the image looks with crop, dithering and image processing.
Crop will allow you to change the framing of the map. Below I have changed it to manual and adjusted the X axis to frame it better.
Dithering adds white noise to give a more textured look. It can often be best to turn it off for smaller maps whilst it can add details to larger ones.
Then if you turn on Image processing you can fine tune the image.
You can also turn off specific colours, for example the cyan, green and pink did not objectively fit in this map so they were disabled.
