The idea is to contact the local businesses in need of help and establish yourself as someone competent that could get the job done.
We have email templates that will help you establish yourself as a leading expert (we tested them in our close case study with impressive results)
In the event that the business is not claimed, you can offer to claim it for them. If they are claimed, you can offer them GMB Optimization services and Local Citation creation services.
Your efforts will ultimately help the business to reach the 3-pack (Google's top-3 results where every business wants to be).
Check the resources available inside the Videos & Help section.
Regarding the rates you should ask for, most people in our close case study mostly went for a $500 flat fee for GMB creation & optimization services for a $400 flat-flee, and $50 monthly recurring for local citation services.
This worked well for the US market, but you might want to downgrade your prices if you're prospecting in a developing country.