To add rubocop to a legacy project, first grab a .rubocop.yml that specifies your projects code style and then do:
rubocop -c .rubocop.yml --auto-gen-config --exclude-limit 500
Then you’ll want to include the automatically generated todo file into your .rubocop.yml.
Run rubocop. Any violations that you now see will be caused by your config overriding the todo exclusions. Find the cops causing problems using rubocop -D
Then fix them by doing things like:
increasing your Metrics/LineLength in the .rubocop.yml
or perhaps setting the Enabled to false.
By doing this you can relatively quickly add rubocop to a legacy project with settings matching an organisations coding style, ready for you to really start making a codebase better.