How:

Login to blogger.com, select the blog that you edit, click the link and then go to Layout Edit HTML
To be able ngedit overall template, you must check / mark / switch Expand Widget Templates
Enter the following code before the </ head> tag or after the appropriate code } ]]></ b: skin> like this:

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


Then set the post so that the cut off, serch code <p> <data:post.body/> </ p> in your template and add the following code right underneath:

<b:if cond='data:blog.pageType != "item"'>
<a expr:href='data:post.url'> Read More..</a>
</b:if>

to create a truncated post, every time you create post, click on the post where you want the Read More .. appear, and type <span class="fullpost"> . then type </span> in the end of the post. To add this code, when you need to post in Edit HTML mode rather Compose..