You can create a reusable designs by navigating to Designs tab inside the Product Creator. Start with a template, and combine it with your own uploads.
After you create a design, you can apply it to any product you'd like. This makes it easy to quickly get started with the same design on many products.
If a design is already used on a product, it may be locked. This prevents accidental changes to live products.
To edit a design that's already used on a product:
Duplicate the design (easiest), or
Remove it from the product that it's used on