Use Bootswatch.com themes with Drupal Twitter bootstrap
Use the theme bootstrap

Download Bootstrap place it in the bootstrap theme folder as mentioned in the theme page. Update jquery to 1.7 you might have to use the Jquery Update (Use the development version of JQuery Update and not the stable one as the development version has option to update it to 1.7. You can do that by going to admin -> jquery Update 

By default the theme uses the bootstrap.css So you can simply replace that css file with the new one you download from bootswatch and you are good to go...