you want to add a post-specific META description tag to your blog posts. Just put a meta tag
inside the head of posts. Go Layout/Edit Html then insert between <head> and </head> tags this line:
<META NAME='description' expr:content='data:blog.pageTitle + ",Put in your common description tags here "'/>
where you replace your own common description tags.
What happens is expr:content= ‘data:blog.pageTitle’ gets replaced by the title of your current page. So when combined with the common description tags, the description tag becomes unique for each page.
This will enable more searchers to find your post.
P.S.: I am not mentioning the second trick about putting title of the blog into the title of the page as recommended on the website above. Blogger already has that feature built in as of today.