Another way is to insert the board into the bag first, then insert the comic afterwards. Of these two methods, I would prefer to put the comic in the bag after the board. It seems that it is much easier to slide the comic into the bag with the board in place.
The third method is to put the comic book onto the board and slide them into the bag together. If you have the board showing a bit on the bottom of the comic, you have much less chance of damaging the corners or cover of the comic from sliding against the bag.