Step 1. Search for "post-header-line" to find this portion of code in your template and add the lines in red as shown.

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

    <div class='post-body'>

      <b:if cond='data:blog.pageType == "item"'>
         <style>.fullpost{display:inline;}</style>
         <p><data:post.body/></p>
      <b:else/>
         <style>.fullpost{display:none;}</style>

         <p><data:post.body/></p>

         <a expr:href='data:post.url'>Read More......</a>
      </b:if>

      <div style='clear: both;'/> <!-- clear for photos floats -->
    </div>

Step 2. Goto Settings->Formatting and at the bottom, you will find the text box provided to specify the "Post template". Copy/paste these lines into that text box and save the settings.

Type your summary here
<span class="fullpost">
Type rest of the post here
</span>