For NPCs or for the farmer?
For the farmer you'd need to go through the mod called Fashion Sense. It has documentation here:
https://github.com/Floogen/FashionSense/wiki/Creating-a-Content-Pack .
I've never used it so I can't give any advice. Common things to do besides read and follow the documentation include download a couple of existing mods, and look at their code and sprites to see how things are set up. Farmers are tricky because the outfit/hairstyle has to work with far more poses and options for sprites than any NPC.
For existing NPCs, it's something you can do via content patcher.
https://stardewmodding.wiki.gg/wiki/Tutorial:_Adding_Seasonal_Outfits_via_Appearances.
(For new NPCs, it would be the same process but there are a whole lot of other character creation steps to work through first.)
For the sprites and portraits, the art side is all yours to figure out, but If you look inside a mod that uses seasonal outfits (or even inside the unpacked base game code) you'll see the general size/shape for the sprite sheets and portraits, and how to display different seasonal or indoor-outdoor outfits.