Start writing or planning in a non-linear organizational fashion - start in the middle, at the end, wherever. Then, when you have the ideas or the plan solidifies, fill in the details or the missing parts.

Gather content -  start with the bulk of information you already have. This might be a brochure or website that tells the important parts of the story. Have the idea in your mind but not all of the details worked out yet?
Every project will have unknown aspects that will get worked out at some point, but that doesn't have to slow you down in getting started!

Start at the end. Start in the middle. Place in filler for the parts you don't quite have or add it in when you have finished the rest. Use imagery you already have finished, and placeholder imagery if you don't have it yet. This allows an iterative development approach for both visual design and with text content. 

Visualize your messaging effectively with the quick view / visual view mode. This shows a detailed representation of how the final published version will look and feel. 
Incorporate text, still images, and pre-shot video or AI generated placeholder content.
Add your own background music or record your voice as a voiceover. Keep in mind social media format can be silent-by-default, so projects stand on their own without sound.

Get unstuck and streamline your content creation process!