If you haven’t figured it out yet....

https://www.digitalocean.com/docs/databases/postgresql/resources/supported-extensions/

There really are 2 ways to do this.

1) Connect to your droplet using psql and then use the CREATE EXTENSION....Command

OR

2) Install PgAdmin4 (locally) , connect your cluster, expand your db, right click on “Extensions”, click “CREATE” and then “Extension” (or something like that) and the dialog that comes up has a ton of extensions to choose from in the drop down.
Be sure to look at all the tabs and all the drop downs to ensure you are getting what you expect (e.g. there may be more than 1 version of postgis available....

Hope this helps.