Doing SEO for others

Hey, so I’m looking to do SEO for businesses, mainly local, and help them generate more leads and thus revenue. I know about the main parts of SEO and have been studying for the past couple months. What is the best way of acquiring clients to work with without having any social proof. I’ve been thinking of offering my services for free to get my foot in the door. Any other options would be much greatly appreciated. Thank you!