Show me how to meet Guidelines/Product Dimensions as part of design in Photoshop

I am new to Photoshop having just downloaded it. I want to start a new store on Etsy however it will require knowing how to design various products such as Clothing, Pillowcases, Wall Art, etc. I have a strong interest in art so I would like to develop the digital skills and then give people the opportunity to buy my designs, art. Once the designs are provided to the production partner the goods can be manufactured. I have delivered a few but these have been through trying rather than knowing how to set things right in terms of: [login to view URL] and the creation in the artofwhere site. I need someone to explain to me how this should actually work and how I ensure I stick to the guidelines with no errors coming through once submitting for production. You may need to register an artofwhere account to use their 3D modeling. Appreciate someone who can explain to me tomorrow ideally.