This isn't the easiest hack to install in a Blogger template. In fact, it took me a few attempts to get this right the first time I installed it! So I would advise you to remember two important things:
Back up your existing template before making any changes whatsoever!

You should always back up your blog template before making any changes to the template code. It is even more important to back-up before installing this hack, as we are dealing with the display of your blog posts, rather than a simple edit to your styling code!
Read each step fully, familiarize yourself with the code, and then read it again!

It is important to pay attention to detail when installing this, otherwise you may place code in the wrong place and get that dreaded "Your template could not be parsed" message.

Also, there may be slight differences to the code which you will be looking for in your template. I will point this out where appropriate, along with the alternatives which may be present in your template.

Here is my step by step guide to installing the selective post summary hack in your blog template:

Back up your existing template code!

To do this, go to Layout>Edit HTML in your Blogger dashboard, and click on the link which says "Download full template". This will allow you to save a copy of your existing template in XML format which you can use to restore your blog if ever your template edits go awry.
Choose to create a new post. On this page, click on the "Edit HTML" tab so this becomes highlighted. Then in the content box below, paste this entire section of code, exactly how this is written here: