Watch the I Don't Worry online guitar lesson by Corey Congilio from Blues Guitar Fakebook: Rhythm Vol. 1
"Sittin' on Top of the World" is a great blues song with a not so typical progression. First off, the form is a 9-bar phrase which will feel odd at first but will become comfortable after a few passes. As with many of these examples, the first pass is the simpler of the two. The second pass will introduce more inversions and ideas I'd use on the bandstand. Note that I use a Bb7 in the second pass and a Bb major in the breakdown. What can I say? I love to show as many options as I can. I don't want to confuse you, so use the version that sounds best to your ears and most interests you. Go dig up Howlin' Wolf's version of this too; it's my favorite!