You can either watch the video or you can follow the steps below:
Step 1) In the Creative Suite page click on the "Create a banner" button.
Step 2) Choose banner size. You will notice a pink feature in the right side of the page named "Help Section". If you click there all steps are appeared there as well.
Step 3) Click the blue button named "Start from blank" to start designing your banner from scratch or choose any of the templates.
Step 4) Start designing your new banner as you wish by using the tools on the top of the page. In the help section you can find guidelines of how to design a good banner.
Step 5) Save your banner by clicking the green "Save" button and then "Save as".
Step 6) In the pop up window, name you banner as you wish and then click "Save as". Done :)