As a person that knows nothing about Docker, I follow these instructions step by step Server Setup | Docs
However, I do not know where in the file system to place the docker-compose.yml and .env files. The above documentation says nothing about the directories they should be placed into and their permissions.
Press insert / i first and make sure on the bottom left it says Insert, once your change is made press the escape key then input :wq to save and close the file.
Or try using nano if it is available it is an easier editor to use.