Simply follow the below instructions which, along with other information, can also be found on Google Code and GitGub.
- Go to FeedBurner and login
- Select the feed you want to work with
- Go to the Optimize tab
- Click FeedFlare on the left side
- Scroll down to Personal Flare
- Add your Flare Unit URL
- Google+ Flare Unit URL: http://hosting.gmodules.com/ig/gadgets/file/108330518946726927825/feedflare-googleplusshare.xml
- You can also host the Flare Unit yourself
- Click the Add New Flare button
- Click the checkbox(es) next to your new unit
- Scroll down and press Save/Activate
If you have any questions please as them in the comments section. Enjoy!