Shopify Product help you:
-Create a product on your Shopify store
-Directly adding a template on it at Control Panel
Step 1: Go to Control Panel> Shopify Product > Add new product
Step 2: In Detail tab, fill in the product details, these details is also showed up on Shopify side
Step 3: In Design tab.
Select a Product Base
You can Edit or Add new this product base as well
You can select the Product base you've created or create a new one
Step 4:
Select a template to design for this Shopify product
Align template
--> Save
You also can download your design then replace the image in Detail tab by this image
After creating this Product Base,
when your customer click to customize this product, they will see this design with template at first.
If they like your design, they can add it to cart then
If they don't like it, they can still make their own design like normal