Looking for portfolio advice - thinking of starting freelancing

For a little context, my background is in non profit communications. I have a pretty broad range of experience including in social media management, writing and editorial support, web management, and branding/messaging. I’m really interested in beginning a freelancing business, primarily focused on nonprofits and mission-driven organizations, but I’m not entirely sure where to start. 

Since most of my experience is tied to one organization, I’m thinking about creating case studies around projects I worked on there (campaigns, publications, social media growth, messaging work, etc.). But I’m worried this might come across as unprofessional or make me look less experienced than I actually am. 

Has anyone else been in a similar position or have any advice on how I might approach creating my portfolio? 

Thanks for in advance!