I'm not a skincare expert. But, as you can see from the samples below: You don't need an expert to write an authoritative guide. You need a professional writer who is deeply experienced in research, interviewing, and turning complex ideas into reader-friendly text. In this case, the client provided the topics and the layout constraints and I took it from there. 

Links to other samples: Content Marketing, Print Pubs, SkinHealthMagazine, Coconuts + Salty Kisses, Business Websites, Nation1099.com, Online Travel, Print Travel, Pamphlets, Blogs / Social Media / Lighter Pieces.