To paddle straight, there are 2 tricks:
[1] have the shaft in a vertical plane. This is the main issue.
Row:
No row:
[2] enter the water with the blade rotated rear face turned to the rail. Note that this reduce the paddling efficiency and is not really needed for 10' + boards. It is more a technique for very short SUPs, and to create "anti row", for instance to aim for a mellow section of whitewater without having to change paddle side.
Of course, you must also apply the general paddling techniques: catch as far froward as possible, apply effort in the 1rst foot of paddle travel, get the paddle out of the water at the feet, not beyond.
Note that visualizing a "nail" near the fin cluster may make you better understand what is happening
www.gong-galaxy.com/forum/viewtopic.php?p=43629
As for vids, I found that Halie Harrison has the perfect technique, in this video at 47s and 1mn. Short and compact, and see how she steer the board by angling more or less the shaft:
#t=47