INHALE: Think of inhaling through a straw, saying "oh" backwards, filling down, not up.
EXHALE: actively, providing abdominal pressure as you breathe out. (hissing)
Tips for beginning clarinets: B is for BREATHING! Woodwind instruments are sounded by blowing air through the instrument. On clarinet, the reed needs to vibrate freely to create the sound. Breathing should be from your core: around the middle of your waist, NOT lifting shoulders.
Try this: breathe in ("OH") for four counts, then hiss out forcefully four counts. Try to empty ALL air on these four. Repeat with a 6 count, 8 count and 12 count hiss. Empty ALL air every time. Notice how you have to use pressure to empty, but on the longer counts, you have to conserve a bit. This active breathing is the type to use when playing the instrument. The instrument provides the resistance which the hissing simulates. Now try it again with 2 count inhale and 8, 6, 4, 2 count exhales.
Be sure the core expands on the inhale. You can also practice correct breathing if you lay on the floor with a book on your navel. Raise the book with your breath, not by arching the back or something weird. A second way to check correct inhale is to bend forward from the waist. Place your hand on your abdomen, and inhale. You should feel your core expand. Stand and repeat the inhale, maintaining the same "feel" in your body.