"How does this work, anyway?"
No one teaches you how to work with creatives. Creatives aren't even taught how to work with creatives - but not to worry!
Acclaimed designer Bonnie Siegler wrote Dear Client, a book on how to optimize the creative process. Below is a super-condensed combination of some of her tips and mine: 
The best way to reach me is by email. When possible, my preference is a single client point-of-contact who consolidates feedback and communicates goals on behalf of others who might be involved. 
Send details such as timeframe, audience, distribution method and your intended look/feel. Let me know if you'd like to replace legally risky images with free/low-cost alternatives.
Billing hourly puts clients in the driver’s seat, since revision cycles are up to you. Sending content in its final form also saves time and money!
When giving feedback, it's helpful to describe the issue ("I'd like it to feel sunnier") than prescribe a specific solution ("Make it all yellow").  Also,  crowdsourcing feedback can leave you with conflicting but well-intentioned advice that drives projects off course and off budget.
Want to edit?  Designers use software most people don't have. While I can build a project in Google Slides, the design capabilities are limited and the results may not meet your needs.
Most importantly, ask questions. Lots of them! Please!