Start by creating a new Rails app using a PostgreSQL database.
rails new yourwebsite --database=postgresql
Create a database.
rails db:create
Run the ActiveStorage installer to install ActiveStorage.
rails active_storage:install
After that, add the following line to your Gemfile:
gem 'spina'
Run bundle install and run the installer to install Spina CMS.
rails spina:install
The installer will ask a couple of questions to help you setup your website.
(Re)start your server and access Spina at /admin.