Make sure you have uploaded any images you want to use first.
Click on the different sections on the left to create your listing (you can move between them in any order)
Save your listing before leaving this page, or you will lose all your changes.
You can Save at any point.
When you have finished, click on "Toggle Source / Preview", then copy and paste the code into
the "Describe" section on the eBay website (Select the HTML tab, not Standard)