Write a relevant short description for each separate page.
Use keywords and keep your description fluent grammatically.
Summing up keywords is considered as spam.
Maximum 200 characters should be enough.
According to the latest polls the metatag description is not used by google to make up the rankings.Other search engines use the metatags so It’s better to provide a description.
Also write for each page sets of keywords and separate them with a komma.Think about keywords that a potential customer would type into a search engine and use that as a keyword.
Maximum 20 keywords should be enough.