First of all, to use the portraits feature, you need to have the framework mod "Farmer Portraits" installed. It's recommended to disable the sample pack: while it should not cause errors, it is redundant with this mod.

Second, neither this mod nor any other I know about nor the vanilla game will generate the portraits for you. (Well, the Farmer Portraits default setting has your sprite enlarged to fill the portrait box. I'm assuming if you want to use this mod, you want something more substantial than that. There is a portrait maker at https://jazzybee.itch.io that you can use for this: it allows you to save the portrait in the exact right size and matches the vanilla portrait art style reasonably well. It only has so many options and doesn't match the diversity that Fashion Sense content packs offer, so it might come down to you doing your own pixel art anyway, at least to edit the generated portraits. (That's what I do)
 
Anyway, here's the sequence of what you need to do:
1. Open the mod folder, enter the /assets folder, and either copy one of the sample portrait folders ("SampleName" and "SampleTwo") then rename it into your farmer's name, or just create a folder with your farmer's name. (You could also just rename one of the sample folders without copying, but given you might want to use them for reference later, it's not really recommended)
2. Put a file named "portrait.png" into it. You can use one of the sample portraits for this, or a portrait generated by the portrait maker I linked above, or a random slug photo; but to start with, it's probably best to actually put the portrait of your farmer that you want to use as a base / default there.
3. Add any other portraits you already have ready and want to use for your farmer, named in the "portrait_[something].png" format. You can use the naming scheme from the SampleName folder, or you can use any custom postfixes you want and set them up in config later. Note though that your custom postfixes will also be your Fashion Sense outfit names, so they can't be too long: anything longer than 18 symbols will just not fit on the wardrobe screen. Don't worry about adding all the portraits you want to use ever, you can add more at any time.
4. Launch the game!
5. Open the mod config menu, find (CP) Farmer Seasonal - Outfits And Portraits, and set the mod up to use the portraits you have / want to use when you want to use them. Note that missing portraits that the mod is trying to load (other than portrait.png which should always be there) don't cause errors, you're simply left with the next highest priority portrait.
6. If you want to use the outfits half of the mod too and haven't gotten to them yet, now's the time to look at the ---OUTFITS--- section of this readme!
----if you end up with more FS outfits than portraits, you can simply copy the portrait file that matches closest and rename it into the FS outfit name. Nothing stopping you from having a folder with 10 identical portraits that differentiate by the color of the skirt set up in the FS half.
7. At any later time you can add more portraits and change the ones you have around, and since 1.6 they will load dynamically during the game after you change the outfit you're supposed to wear back and forth (for example, go to the mines then leave them).