1. Submit your blog url to Flattr https://flattr.com/submit
2. Copy the html code that is generated to your clip board (mark it and press CTRL-C, or CMD-C)
3. Go to your blogger dashboard at http://www.blogger.com/home. It will list all your blogs.
4. At the entry for your blog, click on "Settings" (or what it is called in your language), second-next to the blue "New Post" button.
5. Click on the "Layout" tab, and choose the "Page Elements" sub-tab
6. Click on "Add a Gadget" (you can choose above, on the side menu, or at the bottom. I chose to put it in the site menu).
7. Click on the "
8. Paste the flattr HTML code from step 2 into the content box, and add some title and explaining text if you like.
9. Click save.
Now this will show the Flattr button on every page of your blog. View your blog to see that it actually works.