First select the artwork image. This is the generic artwork which can have various products.
Add the respective title and description for the artwork.
Select the relevant categories and add tags as well.
In the product section of the artwork, you can choose specific images for the products. Ex: A sticker image can be a die cut version of the artwork with a 3mm white border. A poster can be a rectangle image of the same artwork etc.
Add relevant description or any extra information for the repsective product.
Try to have the right size image of maximum width 400px.
If possible, try to optimise your images at TinyPNG
Try to add artwork which people would love to buy! You can always research on what's trending.