What do I do with the NBT file?.
Use it like a .schematic file.
How can I use the NBT file?
You can use it with programs like cubical.xyz, MCEdit and mods like Schematica/Litematica.
You can also import them into your game directly with a structure block - although it might require a redstone power source (#100. It might be a better idea to use a datafile instead - its easier and gives more colors.
...but I need a .schematic file!
Import it into cubical.xyz or MCEdit and export as .schematic.
You do not need a .schematic file for use with Baritone, just use Schematica and the "#schematica" command.
But MCEdit doesn't work
Make sure you're using MCEdit Unified and importing the nbt as a schematic, NOT loading it as a world.
Why is there a row of extra blocks? How do I align the map?
The extra row of blocks (commonly known as a noobline) exists to shade the map properly - without it your map's topmost line will have incorrect colors. You can find the correct place to place your map through trial and error or by using the findCorners(x,y) JavaScript function on MapartCraft, where x and y are your current coordinates. North is always the up-direction on maps and you shouldn't rotate the schematic.
Using cubical.xyz to convert NBT files to schematics is very buggy. Block IDs are called incorrectly by this tool, and the entire thing is instead filled with void blocks. As such, it is very worth learning about how to manipulate structure blocks. Not only is it more reliable, but it's faster and easier on the server's processor than WorldEdit. When ASyncWorldEdit took up to 10 minutes to import the entire schematic file with the void block fillers, importing the NBT file with a structure block only took a maximum of 30 seconds!
First attempted in 1.16.4, I found that the old storage location of structures in ./saves/world/structures has been moved to ./saves/world/generated/minecraft/structures/ (on a server, this is instead ./world/generated/minecraft/structures/). To understand how to operate structure blocks, Pixlriffs made a quick tutorial in 2017 that is still relatively correct. As stated before, though, structures are now stored in a different place, AND structure blocks are executed via a redstone signal, where as the structure is only prepared via the structure block GUI.
The process of using structure blocks is super simple. Create the NBT file with the mapartcraft. Stairstepping and dithering are encouraged for shading and color matching. Copy the NBT file to ./<world>/generated/minecraft/structures/, then in Minecraft, enter OP and switch yourself to Creative mode. /give <username> structure_block 1 and place it in the exact north-western corner of a map tile. Enter the structure block GUI and set it to load, then type the filename (minus the ".nbt") into the interface and click Load. Note - if the structure block is too low, the structure will merge but not replace any already-existing blocks which, in the case of a a landmass, will cause a mess of trees to appear throughout the map. Therefore you will need to use WorldEdit or manually clear a perimeter for the map (or simply place the structure block up above the landscape, provided that the final stairstepped map does not exceed the world height limit). After the map is loaded and queued, install a button on the side of the structure block and click it. The game may freeze for a few seconds as the structure is read and pasted into the world, but in a short time, the map will simply appear to the southeast. /give yourself a blank map, fly over the map tile and right click to create it. Fly to the corners to fill everything in and verify that it worked. That's it!
What is a map.dat file?
It is the native format Minecraft stores map data in, meaning you can use it to import maps into your worls without needing to build a physical structure.
It also enables you to use an extra shade of color not usually accessible.
What do I do with the map.dat file?
You can use the map.dat file in singleplayer or a server you own.
Create an new map in-game, go to your world's save file, then the data folder and from there you can replace map_xxx.dat files.
Map size?
This will define how many maps you will create for your picture.
When creating bigger maps, it's recommended to split it into multiple schematics. This can be done with the "DOWNLOAD AS 1X1 SPLIT" button, which will download your map as separate 1x1 schematics.
My image is stretched!
Change your map size, enable the crop option or edit your image with an image editor.
Staircasing?
This will make your map 3D. Doing so will give you 3 times the colors, but it'll also make the map a lot harder to build, as it is not flat.
Better color?
This setting will give you more natural colors. Disabling this will make the website faster and give you slightly worse colors. It is recommended to keep this enabled.
Dithering?
This will add grain to your image that will make it look a lot smoother. It is usually recommended to disable this for things like flat-colored artwork.
Floyd-Steinberg dithering is the most accurate one, but the Ordered/Bayer dithering will usually have less artifacts and gives the image an unique style.
Add blocks under/Block to add?
Here you can pick the block that will be put under either important blocks (eg carpets, sand, pressure plates) or all blocks. This block will also be used for the noobline, which cannot be disabled.
Presets?
You can use presets to save and load block configurations. Pick your blocks and click "Save" to save them as a preset, pick a preset to load it and click "Delete" to delete the loaded preset.
It is also possible to share a link for your preset with others.