Log into your creator profile and make sure 'your Shop is set up and published'. Now click 'My Designs' on the left and select the shop which you want the design to go to from the top of the page (if you have more than one shop) and go to 'Add new design'. You can give your design a title here straight away or later on if you prefer. Then click 'Open Editor' and upload your design or create personalized versions of it with the tools on the left and right.
If the design is ready click 'save and exit' up on the right corner. To add it to your shop you have to set it to 'Publish' by clicking your design from the 'My designs' page and
1) add a title if you have not already
2) agree to the terms and conditions
3) add the products you would like to offer your design with (by scrolling down and selecting)
4) finally click 'Publish' in the right corner.
You can also add an optional description to your design and add specific tags if you like. And if you're not ready to publish just yet, click 'Save changes' on the upper left corner and just come back to it later.