Keep in mind that a good description will improve your CTR in Google too. That said try to write something attractive and use your keywords a couple of times. Check your competition and you will find out which descriptions do well in Google (keyword density, etc.)
you will have to write description for search engine as well as user. target your keywords properly (don't overstuff them). write an enticing description with proper grammar which can be well read by the user.
Give a short and crisp description about that page for visitors as much as possible. It should be unique. Do not put more keywords on it or it becomes spammy and user get disappointment.
When im writing a page description, im writing down first all the keywords (based on its searchability); from then, i come up with descriptions using those searchable KWs.
unique description with rich content with your page related. don't go through off topic of that page.
I tend to disagree with it, now days nobody reads description, if anything can have a effect on CTR, that's the title