To decant the potion into the flask, one must have at least 6 doses of juju hunter potion in their inventory, as well as one or more potion flasks. Using the flask on the potion will open a menu to decant them. The optimal inventory setup for decanting potions is 17 4-dose potions and 11 flasks.
Once the potion has been partially drunk, no more doses can be added to it. After all six doses have been consumed, the flask will shatter and disappear.