One trick of the trade used by all developers is to save a back-up. This way, if you can't undo your changes, for whatever reason, you can still revert to the original. After saving a back-up to your harddrive find the following piece of code: all-head-content
(In Windows do a CTRL F)
It will take you to the following:
<b:include data='blog' name='all-head-content'/>
Just after the above code add the following.
1. Tell Blogger which page you want to give the unique meta tags.
2. If you want to use the META Description Tag describe your content in a sentence or two.
3. If you want to use the META Keyword Tag put in your keywords and separate them with a comma.
<b:if cond='data:blog.url == "http://yoursite.blogspot.com/someYear/someMonth/fileName.html"'>
<meta content='How to add different meta tags for different posts' name='description'/>
<meta content='meta tags, blogger' name='keywords'/>
You can repeat this step as often as you want to.
It is my opinion that using these tags is not worth the effort. Ten years ago, yes, they were useful. Today, after all the abuse by SEOs, these tags are close to useless.