Everything is pretty much explained here. This is a demo application, do not bindly copy-paste to your app, there are some dangerous things in here.
You need Ruby and Sinatra for this app to run.
# Once you have ruby and rubygems installed :
$ gem install sinatra # sudo may be required, depending on your setup
Make sure you configure the global : $ACL, $BUCKET, $AWS_SECRET. Then :
ruby app.rb