To subscribe to outside communities on the instance that you signed up on, go to the search bar and type in the name of the community @ instance with an exclamation at the beginning.

For example, if you want to follow the Jokes community on lemmy.ml, you would type into your search bar: !jokes@lemmy.ml. It will pop up and you click on it, which will take you to the community. Click on where it says sidebar and you should see the plus sign. You do this for all the communities you want to sign up for that are not apart of your instance.