The very first job is to open a new blank document set up to the dimensions of an Etsy store banner, which at the time of writing is 760px by 100px deep. I would advise that you confirm these dimensions before pressing on with making your own banner.
When you first launch Seashore, you are presented with a New Image dialog and make sure to enter the Width and Height as 760px and 100px respectively. You can ignore the Resolution setting as this has no effect on how your banner will display online, but do ensure that the Mode is set to Full Color, unless you're going to produce a black and white store banner.
When the settings are correct, click the Create button. If Seashore is already running, going to File > New will display the New Image dialog and the settings should be entered just the same.
Next we'll change the background color of the Etsy store banner.