Comment your code in a way that makes it quick and easy for a customer to not only figure out what you have built but also to allow them to effectively modify your file to their needs. Remember, you are selling to a broad range of technical abilities. At least one comment per function is usually enough, but if it is a complicated concept or there is a lot going on, you should have more.
Make sure your comments are descriptive. If you have a function that runs when the user clicks the left mouse, for example:
//This is the function that runs when the user clicks the left mouse