At the moment Black Feathers only spawn as random drops on the forest floor. They share a spawn pool with other little "junk" items such as Eggs, Forest Mushroom, etc. So you have to pick up those items from the forest floor before new ones can spawn in next day. The type of item that spawns is random.
You can't get into the Human Country, it's not implemented yet and probably wont be for a while. Currently I'm working on the Cave system where the Werewolves live. After that I plan on focussing on the Witch Domain.
I haven't made a walkthrough because it would take a significant amount of time as there are a lot of features that would need to be explained in detail and I think that working on more game content is probably preferable for most people (in the long run at least).
There is a Discord server for the game where there are quite a few people who could give some good advice if you feel stuck and need a quick reply. The link is in the post bellow this one.
There is also an extensive thread for the game on F95Zone but searching for specific answers in it may be rather time consuming.