First got to http://panzi.github.com/Browser-Ponies/ and scroll down to the list of ponies. If you don't want to use the Mane Six click on "Set all Ponies to 0".
Then choose the ponies you want.
Then scroll back up to where it says "Or paste this into your webpage:" and copy the contents of the text box.
Then sign in to your blogger account and click "Design" on your blog.
(In case "Template" is not already selected click it.)
Then click "Edit HTML".
Then click "Proceed".
Then paste the embed code somewhere after "<head>" like shown in the image and click "Save Template".
And Ta-Ta, you've got ponies on your blog.