Installing Spina is a matter of adding a gem and then running a few commands:

bundle add spina
rails active_storage:install
rails spina:install

You'll see a load of files being installed, most of which are migrations. At the very end you'll get a few prompts to name your app, add your email, and finally a password.
Note: If you get an error running spina::install, be sure to specify the version in your Gemfile, for instance 

gem "spina", "~> 2.18