I am at the level where I am writing something non-trivial for the first time with elixir and phoenix. I did my first MVC application in Chicago Boss, an early all-erlang framework. When I'm learning something new I always try to solve a problem I see in my travels as a consultant. That way the customer requirements are fresh in my head when I am designing. This time, I'm writing a supply chain management system that explores a combination of eager and lazy architecture to help solve the stresses of a large, distributed organization.
My approach this time was to take Dave Thomas' programming course https://pragdave.me/ to get me to the level where I could think in elixir and phoenix. Then I ran through the course a second time but wrote my own api along side of Dave's videos. Once I had the api worked out I turned to the guides and books for ecto and phoenix. If you're starting out fresh with elixir and phoenix, I recommend this approach.