The Hufflepuff Common Room is located in Hufflepuff Basement. The entrance is located in the kitchen corridor, past a large still life painting that conceals the entrance to the kitchen, hidden by a pile of large barrels found stacked in a stone nook on the corridor's right-hand side.
To enter the room, one must tap the barrel two from the bottom, middle of the second row in the rhythm of 'Helga Hufflepuff.' If someone taps the wrong barrel, or taps a barrel an incorrect number of times, this results in one of the lids of the other barrels bursting off and drenching the person in vinegar.
Writing by J.K. Rowling
"A sloping, earthy passage inside the barrel travels upwards a little way until a cosy, round, low-ceilinged room is revealed, reminiscent of a badger's set. The room is decorated in the cheerful, bee-like colours of yellow and black, emphasised by the use of highly polished, honey-coloured wood for the tables and the round doors which lead to the boys' and girls' dormitories (furnished with comfortable wooden bedsteads, all covered in patchwork quilts)."
"A colourful profusion of plants and flowers seem to relish the atmosphere of the Hufflepuff common room: various cactii stand on wooden circular shelves (curved to fit the walls), many of them waving and dancing at passers-by, while copper-bottomed plant holders dangling amid the ceiling cause tendrils of ferns and ivies to brush your hair as you pass under them."
"A portrait over the wooden mantelpiece (carved all over with decorative dancing badgers) shows Helga Hufflepuff, one of the four founders of Hogwarts School, toasting her students with a tiny, two-handled golden cup. Small, round windows just level with the ground at the foot of the castle show a pleasant view of rippling grass and dandelions, and, occasionally, passing feet. These low windows notwithstanding, the room feels perennially sunny."