If you have enabled federation on your instance, all you have to do is go to search and put the community url of the instance and search for it. Your instance should be able to fetch that community and show it to you. After that, you can subscribe it from your own instance.
Hope this helps :)
I guess for that both the instances need to explicitly allow federation in settings. Once it is done, both instances will be able to see community from each other.
However I am new to this just like you so possibly the veterans here might know more than us :D