Navigate over to the products page and view the table of products.
Find the product you wish to make custom, and click the grey 'Standard Assets' button located at the end of the row.
This will now mark the product as "Custom Asset" (green button), so when an order is created containing this product, we first send the customer a 'Preparing Asset' email, and allow you to upload a custom asset to the product for that order once the asset has been created.
You can also upload other assets to this product, so when you send the custom download email to the customer once the custom asset is uploaded. The customer will download not only the custom asset but all other assets for that product.
This feature is not available on the Starter plan
