I want to write a blog post building upon someone else's paper, how can I properly cite/credit them?
You have two issues to deal with. If you properly cite and attribute the ideas to the original author, you avoid plagiarism issues. If you don't copy too much, but properly quote, from the paper then you avoid copyright issues. 

But, ideas are free to use and to adapt. Simplifying what you find in a paper is a good thing to do. 

One doesn't obtain ownership of ideas by writing a paper. 

I therefore see no ethical issue at all with what you suggest.