This tutorial explains how to display an advertisement under post title in a Blogger blog posts. To add a ad banner just below post title, first you need to go to "Edit HTML" page of your blog.
After that find this code (You will find it 2 times):
Now paste your advertisement code just below above code (you have to paste it twice, because above code available in 2 places).
Note: If you need to display your advertisement only in post pages, wrap your advertisement code like this:
<b:if cond='data:blog.pageType == "item"'>
Your Ad Code