When you click on Print Settings in Photoshop's Print menu, then click on Paper size, scroll to the bottom to Custom Paper size. Click "+" to create a new preset. In that dialog create a 12x18 inch new preset with the margins set to 0 on all 4 dimensions. Give it a name (say change it from "Untitled" to 12x18) and revert to Print settings. Because it is 12x18 on a 13x19 sheet you will want a half in margin on all 4 sides. In the Photoshop Print menu, make sure the box called "Centered" is checked, then go back into Print Settings and make sure your new preset is still active. If so, exit from there, click Print and it should come out fine.