First of all, you must enable embedded comments in the Blogger menu. To do that, go to Settings > Comments and on the Comment Form Placement select Embedded below post. Then, go to Layout > Edit HTML and check Expand Widget Templates, then search for code that looks like this:
<p class='comment-footer'> <b:if cond='data:post.allowComments'> <a expr:href='data:post.addCommentUrl' expr:onclick='data:post.addCommentOnclick'><data:postCommentMsg/></a> </b:if> </p>
and replace it with this:
<p class='comment-footer'> <b:if cond='data:post.embedCommentForm'> <b:include data='post' name='comment-form'/> <b:else/> <b:if cond='data:post.allowComments'> <a expr:href='data:post.addCommentUrl' expr:onclick='data:post.addCommentOnclick'><data:postCommentMsg/></a> </b:if> </b:if> </p>
Save it, and you should have the comment field embedded under the posts, like we have here, on WebUpd8l.