I would suggest taking off the weather stripping. Then closing to door and take some measurements of the space between the door and the garage floor. Cut some filler strips from the same dimensional wood as the garage door, you will be making two wedges. After you get your wedges cut then fit them to the door and predrill holes in the wedges and into the door with a small drill bit. Now take the wedges and paint them on all sides. After the paint is dry secure them to the door with some decking screws, then reinstall your weatherstripping. Close the door and admire your handy work. Shouldn't be to hard of a job but remember to sceure the door when you are working on it so it doesn't close on you. Good luck.