Step 1:

1. Select Layout > Edit HTML > Edit Template.
2. Tick on Expand Widget Templates.
3. Search for below code.

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

4. Delete above code & replace with:

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

5. Save Template


Step 2:

1. Select Posting > Edit Html
2. Put the content that you want to include in the read more between <span class="fullpost"> </span> tags:

<span class="fullpost">

Put here

</span>

3. Publish Post