How does that work?

Do the pantsu have a hole in them that you thread your tail through when you put them on?

For magical tails that clip through everything, probably, yes.

A more practical way to do this would be to have a notch where the tail goes and a clasp/tie on the back of the waistband so you just pull them up and secure them around the base of the tail, instead of trying to feed feet of tail through a hole every time you change undies.

Constantly leaning forward while also holding those water bags up all the time must be agony on her back.