Etsy mockup for use on my website

I need help!  I am using Printify, Shopify & Placeit.  They don't have the t-shirt I need.  I found one on Etsy, purchased it...I don't know how to get my image (already formatted) onto that Etsy mock up so I can attach it to Shopify so it appears on my website.  Any ideas?  I think I'm half way there I just can't finish it up.