The main mod folder (the root) contains the "assets" folder, the "content.json" and "manifest.json" code files, this "readme.txt" file that you're reading right now, and possibly the "config.json" file if you have already launched the game with this mod in it at least once.

- manifest.json: the only reason you should ever have to touch this file is if I messed up the version number when updating the mod and you need to edit the version number in it to make SMAPI / your mod manager shut up about an update being available when you've already installed it. To do this, open the file, find the line that says "Version: "(some number)" and make this number match the version number in the downloads section of the mod page (https://www.nexusmods.com/stardewvalley/mods/19503?tab=files);

- content.json: this is the file you might want to edit to add custom mining, desert ot bathing locations or custom festivals. The instructions for how to do this are all inside the file, and can be found by searching for "//add custom mining locations", "//add custom desert locations", "//add custom bathing locations" and "//custom festival code sample" respectively. There is no other file you need to modify to make these changes;

- readme.txt: the file you're reading right now. Hi!

The assets folder should contain the "_code" folder, the "background.png" file and two sample portrait folders: "SampleName" and "SampleTwo".

- _code: !!!theoretically there are no circumstances under which you will need to interact with this folder in any way whatsoever!!! But just in case you're an intrepid coder who wants to make a personal change, here's the quick rundown. You'll notice that there is the same trio of "setup", "festival" and "mining" repeated twice times. One is for the outfits and one is for the portraits. The "setup" files are the ones called directly from content.json, and contain most of the actual outfit switching code, which has the same structure in both branches - sequential changes in the order in which the outfits overwrite each other. The "festival" files are separate so I can make a layered "if" check for DayEvent, and yes, it didn't work otherwise. The "mining" files are separate just because the code there is a bit of a pain in the ass. Yes, inside each file the same code is repeated six times with different conditions when just three would do; it's for readability. If you want to make a change that isn't fixing a typo I made, you'll have to make it at least twice, for the portraits and the oufits. Good luck;

- background.png: this mod does literally nothing with this one except load it, feel free to replace this file or never turn it on in the Farmer Portraits settings in the first place. Honestly, I expect you to. It's just there to prevent errors;

- SampleName: this is a folder that contains example portraits. Theoretically you can start up the game, make a new save with the farmer named SampleName, change nothing in the config, and these portraits will be used. In practice you're supposed to make your own folder named your farmer's name and put your own portraits there, using the portrait names in the SampleName folder as reference. (Copying SampleName and doing "Save As" to replace each of the portraits is a quick, easy and relatively foolproof way of doing that). Upon making changes in the game's config you can rename the files to whatever you want, though note that all portrait files should still start with "portrait_" after renaming.

- SampleTwo: this is a folder serving as an example of a minimal setup, when you just have one portrait for all occasions. Just make a folder with your farmer's name, put a "portrait.png" file into it and you're done. You can add more portraits later at any time.

- add more folders for each of your farmers! You can use the same folder for multiple saves with the same farmer if they share a name, or you can make a new folder for each save if the farmer has a different name in each. As I explained in the SampleName folder description, you can copy it and use its file names for reference, or you can give your files custom names that follow the format of "portrait_" + the postfix you set up in the in-game config (see the ---CONFIG--- section, or just figure it out in-game) ("portrait_base", "portrait_summerRain", "portrait_eveningDress", etc.)
-- each portrait file must be a square .png file at any resolution. Transparency is supported, have fun & go nuts;
-- there MUST be a "portrait.png" file in each folder. It will be overridden by any other portraits you use and you might never see it in-game ever if you use the seasonal outfits feature, but it must be there, or the mod won't work. Just copy some other portrait under that name, or keep the sample portrait.png, or use a random slug photo, it doesn't matter. It just needs to definitely load to give the other portraits something to edit;
-- it doesn't matter what else is in these folders than the files the mod will be looking for. Put as many sketches/WIPs/alternatives under different names there as you want. But:
-- UPON UPDATING THE MOD THE CUSTOM PORTRAIT FOLDERS WILL LIKELY BE LOST. Always back up all the portraits before updating, or better yet, store them elsewhere in the first place and just copy them to the mod folder after every update.