I used to do this many years ago for a living. We would blow in on finished and non finished dry wall. If the joints are not taped and they are not tight then you will get some insulation trying to poke thru. Insulate your walls first, hang & tape, then blow in the ceiling. Your good to go.
Depending on your rafter height, you may have to crawl the whole way or you can walk. Ether way put up some 2X's or 4x's for walking or crawling. You can also use plywood strips 6" wide run the whole length of your attic. This way you wont wind up stepping between the joists by accident and making a nice new hole.
Start to spray at the farthest point in the building, stay in the center and then just point the hose in front then turn to do the other side. Always spray the eves first then work your way back to you. Do not cover up the eve vents. You can use the actual roof as a back stop so your not spraying the insulation into a big pile & compacting it. You do not want to compact it in. It needs air and lift to work properly. Also take a ruler or stick with the height marked on it so you know how deep you need to spray. If it gets a bit to dusty you can sprinkle a small amount of water in the hopper to keep the dust down. Just do not pour a lot on it. Just a quick spray with the hose, one pass over, or a few sprinkles from a gallon jug.
Good luck, where long sleeves and respirator or mask.
