Upload your design here, confirm you dimensions, pick a material, and add to cart.
Rather than proceeding to checkout, exit the cart by clicking the little 'x' at the top right
Click on 'design' at the top left of the page, then on my designs to bring up your files:
Select your file and update your material (it will have defaulted to the last one used).
Add to cart again:
You can proceed to checkout from here, or add more parts.