Pheeds.com

This video shows 5 resources for creating designs to use for print on demand products.
 


PlaceIt
https://placeit.net/

Canva
https://www.canva.com/

Photopea
https://www.photopea.com/

MerchDesigner
http://merchdesigner.com/

Fiverr

placeit is well worth buying I've been using that A LOT.  If you buy design stuff you can put on sweatshirts
and other things you really need mockups because the Fulfillment company mockups leave a lot to
be desired in some cases although right now the store i was helping with is full of them and not
selling too great yet.  Don't know if they will pick up.  We have switch some to placeit.

I do recommend a good mockup company if you need one it really makes a huge difference altho you can
get by if you don't have the income to do it.