For Dot Matrix printers, horizontal alignment is usually controlled by sliding the pin-feed wheels right or left on the carriage -- starting with them centered. If you can't center them, for instance if the left-hand wheel is fixed, use the left/right adjustment in Shellstock Tags Setup to move everything to the left an inch or two.
Make sure you have the Tag width in Setup > Printers > Tag Printer width set to 8.5 inches to start, and that the pin-feed rollers are centered in the carriage if possible. Enter the distance you want to move the printing in inches the 'Tag Left/Right' box, then select either 'Move LEFT' or 'Move RIGHT', then click 'Save Changes'.
