I have just swapped from the old 2 litre Osprey bag with a screw top to the more complicated 3 litre Osprey bag with the wide top and detachable drinking hose.
The old one was a cinch - detach, strip and clean the bite valve assembly, wash out bag and lid with hot water, hang upside down for water to drain to bottom for an hour or so, pour out excess water, pop bite valve assembly into bag so you know where it is, loosely attach top and chuck it in the freezer until next time.
The new bag is more complicated.
Detach drinking hose, strip and clean bite valve assembly with hot water, flush out hose. Put to one side
Drain bag and flush with hot water.
Reattach drinking hose to open non return valve and flush out the hose that is attached to the bag,,
Invert bag and open bite valve to drain out both hoses and non return valve.
Hang bag upside down, slide the bit of plastic that seals the bag shut into the mouth of the bag so it can drain properly.
Leave to drain, ensure bite valve assembly and non return valve are both free of water.
Bung it all in the freezer.
From time to time a deeper clean may be necessary, I use Milton tablets, the points of cocktail sticks and the Osprey cleaning kit to dig stuff out. The first time I used the new bag I threw in a Power gel tablet to make the water taste better - out of the tap our water is gopping and the water filter does not hold enough to fill the bag. The tablet discoloured the bag but a soak in Milton cured it.