Then you can choose which type of signature you would like.. which fonts, how large and so on... Go ahead and explore. Once done, just save the signature to your computer and / or copy the HTML code to put inside your blog.
Having done that, go to the 'design' page of your blog, and click on Settings... the on the Post and comments tab
Then next to "post template" click "add" as per the screenshot below...
A box will open up. Just paste the code you copied earlier into the box and save settings on the top right corner.... It might take a little while but after that... You are good!!!
Happy trying it out!!!!