1) In Google+, hover over the post you want to embed, click the down arrow in the upper right corner of said post, and click "Embed post".
2) You will get HTML/JavaScript code, like the code in example A. Copy it, and paste it where you would like the embedded post to appear.
Example A
<!-- Place this tag in your head or just before your close body tag. -->
<script type="text/javascript" src="https://apis.google.com/js/plusone.js"></script>
<!-- Place this tag where you want the widget to render. -->
<div class="g-post" data-href="https://plus.google.com/116066401701241808979/posts/DLF9DC4KHVe"></div>
For WordPress.com blogs only (the version of WordPress that you don't host on your own server):
1) Find the post you want to embed in Google+ and click the arrow, but instead of clicking "Embed post", choose "Link to post".
2) Copy the URL that you get, and either paste the Google+ URL on its own line like in example 1, or use the official WordPress shortcode in example 2.
Example 1
https://plus.google.com/116066401701241808979/posts/DLF9DC4KHVe
Example 2 (replace the existing URL with the URL of a valid Google+ post, and use square brackets instead of curly braces)
{googleplus url=https://plus.google.com/116066401701241808979/posts/DLF9DC4KHVe}