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):

<div class='post-header'>
<div class='post-header-line-1'/>
</div>

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
</b:if>

Look at this image to understand it well:

Save your Blogger template.