I'm new but let me tell you what I know so far.

First, you need to find the identifier for the community you want to follow (for example, the community we are in right now is !asklemmy@lemmy.ml) from the sidebar. Copy it, go to your instance (feddit.it) and paste it in the search field. You should find the community as a result there and a button to subscribe. Another alternative is to go to the link https://YOUR-INSTANCE/c/COMMUNITY-ID-WITHOUT-THE-BANG (example: feddit.it/c/asklemmy@lemmy.ml). If it does not appear maybe this specific federation has perhaps been forbidden by the owner of feddit.it