Most of us have already added that about author widgets in our layouts which is provided by Blogger officially.That widget does shows your Google + profile but let me remind you that the pic size should be 128px by 128px otherwise it may not show.
For those who doesn't added their profile on Blogger then simply follow the steps
- add this line in your your layout (anywhere)
<a href="https://plus.google.com/GOOGLE+ ID"
rel="author" target="_blank"> Join me on Google+</a>
- Now replace GOOGLE+ ID.
- you can get it by the following method.
https://plus.google.com/u/0/101116903777663777708/The area I've highlighted your Google + ID.
Don't try to play with it.This line will connect your Google plus profile with your blog.