So if you're on the website on your desktop you copy the URL of the instance+community you want to join (an example would be "https://yiffit.net/c/furry" , for their furry community). Copy that URL and go back to your home base instance of Lemmy pawb.social. From there click the search icon in the upper right, paste the URL into the search box and see if any results come up. If they do, click on one to be navigated through your pawb.social instance into the yiffit instance with that community and click subscribe.
If you use the Jeroba for Lemmy app on Android you can just click the menu icon (second from the left) and make a search for the community name (i.e. "furry) and it should pop up with all the instances that have a community named furry.
It's not too bad in Jeroba, but the Lemmy website's default behavior is a lot of steps + searching just to subscribe.
