1. If you don't already have your button designed you can use fotoflexer.com or picnik.com to make one from scratch. They have great fonts and stickers you can use.
You can also go to shabbyblogs.com or graphicsfairy.blogspot.com to find button backgrounds and pics.
2. Just upload these pics to one of the photo sites and add your text. Save them to your computer.
3. Upload your saved pic to www.photobucket.com.
4. I use this site to generate my buttons: http://www.mycoolrealm.com/sandbox/gbgen/
5. On Blogger go to the Design tab and click on "Add a Gadget".
6. Select "HTML/Java Script" and paste the code that was generated from MyCoolRealm.
7. Save your changes and your button is done.
If there are any questions please let me know!