I wasn't following any tutorial, so this was a good bit of trial and error on how much paracord I actually needed, and especially finishing up the bottom side. I tried using hitching and weaving, but just couldn't get anything to work to my satisfaction, so settled on sewing the bottom side ends together. And although the bottom side is not flat, it'll still stand upright, and the needle/thread work is secure.
I used a length of tether cord to attach the canteen cap to the canteen, with scaffold knots/loops to prevent loss.
I had no idea of how much cord I'd need at the start, so after making a loop around the canteen with a few feet of cord and holding it with a cord lock, I started cutting 10 foot lengths of paracord and cow/ring hitching them onto the loop to fill up the space all the way around. I ended up with 36 strands/72 working ends, for around 365 feet of paracord to start with. That left me with using around 6 feet of each 10 foot strand for the way I completed the cover. Other finishing methods would have used more cord.
Using alternating square knots, like I'd done with paracord belts and a guitar strap several years ago, I worked my way horizontally around the canteen. I wanted the 8 inch height of the canteen covered, but you could make it shorter, or possibly longer to close the top, maybe using velcro, fashioning a flap, or sewing in a zipper. The knot work is tight, making for a stiff but still flexible form, so the pouch could be used to carry other items besides the canteen.
My hands sometimes bother me and cramp up, so I took frequent breaks with this project, also being careful to avoid blisters that you can sometimes get with tightening a lot of paracord square knots. I have no idea how much time I actually spent on this project, but there's definitely a lot of hours in this one. I used an adjustable shoulder strap from my Spec-Ops Pack-Rat with the cover, adding about 12 feet of paracord, with a Solomon bar/Portuguese sinnet/cobra stitch, for a wider shoulder pad, knotted over it.
The grappling hooks were made for use by soldiers in the field, to snag trip wires and IED detonation cords, but any good scout or civilian could certainly find them useful to retrieve items that might be out of reach otherwise. If you hadn't seen them before, you now know you need one your EDC key ring! I have another on a split ring and attached to a Drop-Line Rapid Line Deployment System. My deviant side can't help but imagine a cat 'o nine tails fitted with lots of the little grappling hooks, ouch...