Thanks for choosing QuickBooks as your accounting tool, @carol32.
You can enter the plain shirt as non-inventory items. After processing them with graphic designs, you'll have to enter and set them up again as inventory items.
To learn more about adding and editing product or service items, you can refer to these articles:
Adding inventory and non-inventory items (video tutorial)
Meanwhile, I'm adding some more references about QBO. It provides the links and current updates.
Let me know how it goes or if you have any follow-up question. I'll be here if you need further assistance. Wishing you all the best!