Read things out loud. This is a simple tip, but it's so worthwhile. Anytime I'm writing content everyone around me knows eventually I'm going to read it out loud before hitting post.
I've whispered content to myself while getting an oil change. I know, maybe I looked a bit crazy but it's the best way to proofread content. If the content on blogs or Instagram or newsletters is all about being your voice - shouldn't it sound like you? I had trouble with this when I moved from the corporate world into running my own business. I realized - things don't have to be formal as long as they make sense to the reader and the best way to make it sound like yourself is to hear it out loud.