1. Go to https://www.printdirtcheap.com and select the product you need to work on from the left menu or the tab All Products on the top. 

2. Download the file you need from the Template tab. 

3. Open the psd file in Adobe Photoshop and follow the guides, create your artwork in the Place your artwork here layer, make sure that any artwork and details you don't want to get cut-off should stay inside the blue line (safe zone). 

4. Delete the Template Layer when done with the Artwork. 

To Save a Print-Ready PDF

To save or export a psd file to PDF, go to file - Save as, change the Save as type: to Photoshop PDF. Click Save.