I found this link helpful, I will crosspost here:
Use option –skip-webpack-install when generating a new app
Change Gemfile, remove webpacker, add:
gem 'sass-rails', '>= 5'
gem 'sprockets', '~> 4'
gem 'sprockets-rails', :require => 'sprockets/railtie'
Then
bundle update sass-rails sprockets # if you are updating
bundle install # or simply install
If you are using sprockets 4, change app/assets/config/manifest.js to:
//= link_tree ../images
//= link application.js
//= link application.css
If you are using sprockets 3, add to config/initializers/assets.rb:
# Rails.application.config.assets.precompile += %w( application.js )
Restore app/assets/javascripts/application.js:
//= require rails-ujs
//= require turbolinks
//= require_tree .
Change javascript_pack_tag to javascript_include_tag in app/views/layout/application.html.erb