https://dwarffortresswiki.org/index.php/Graphics is a good place to start
You can also look at how other mods or vanilla do it too. It's simple enough to have a static graphic for creature, it can be complicated if you want to start layering them
My advice would be to write a second mod that requires the long night to be loaded before it, rather than directly editing the long night files. If squamous' mod updates it will probably wipe your changes.
There is an excel spreadsheet included with the mod to help with the xy coordinates too

Ah, fair enough. How would I go about writing a graphics file for use of your mod?

I wouldn't have a clue, sorry. It could be this mod, squamous' mod or a combo of the two.
You also need to write the graphics file for it to actually do anything, just putting it in with another mod won't do anything on its own

Oh, interesting.

I was attempting to copy this over to one of Squamous's mods, The Long Night, to see how it would look, but it crashes upon world gen. Do you know why this would be the case?

No actually, this works for graphics mode. I made them as a stop gap when I was converting my mod to the steam release and didn't have proper graphics done yet
It's based on ascii

I take it this only functions in ASCII mode?

Either make this a required mod with your info file, or just download it and copy the files over

How would one actually begin to add this to a mod?

Wow how the heck did this make the most popular for the week list?

Not currently! I don't know what else to add to be honest

There's a readme in the folder when you download it, the alphabet graphics have a cheat sheet for tile coordinates and the punctuation graphics file has the y axis value in the actual png file

That's all specific to this pack though, check out the wiki for generic modding info

Heya are you still working on this? Also do you have a guide :p

New update! I got a bit over excited and now there are 129 new sprites in multiple colours!

I'll see what I can do. I want to push an update to my main mod first and then play it a bit though

If anyone does end up using this for their mods, let me know! It would be cool to know it's being used :)