The way I find feeds is fairly simple:

1. Browse Lemmy/Hacker News/Reddit or just let friends send me articles.
2. When I find an interesting article check the feed and see if the topics seem interesting.
3. If it seems somewhat interesting subscribe.

I tend to subscribe fairly easily, if it looks like it may be interesting I add it. But I am also fairly quick to unsubscribe if I find that I am not enjoying a feed.

This isn't the fastest system but does build up a collection of interesting feeds over time. And if you are starting from scratch you can just lower your "interesting" threshold to subscribe to more quicker, then as your collection grows and grows in quality you can prune them.