Step 1: Find an imageWebsites like Know Your Meme are great for finding meme template images, because they also make it easy to check that you're using it correctly. Meme responsibly - there's nothing the internet hates more than a misused meme. You can also use Google Image search. Be sure to use an image that doesn't have text on it already!
Step 2: Upload and Add Text
Once you have your image, just drag and drop it into the Google+ post box.
Once it uploads, click "Add Text."
Just add your text and click save, and you have a meme!
Note that when you return to the post box, your image may not show its meme text yet. That's okay! As soon as you post the image, the text will appear.