Hair mask
A hair mask is a more powerful conditioner that needs longer to work. While a conditioner mainly smooths the hair's surface, a mask's ingredients penetrate deeper into the hair shaft. Once or twice a week is enough. Use it on towel-dried hair instead of soaking wet hair—that way it won't become diluted.
Moisture or protein—that's the choice. If your hair feels dry and dull but is still stretchy, it needs moisture and you should use a hydrating mask. If it feels rough, breaks easily and barely stretches, it needs protein or keratin. Overdoing both at the same time will work against you: too much protein without moisture will make your hair even stiffer and more brittle.
Leaving it on longer doesn't automatically help. Most masks are ready after five to ten minutes; after that, you're mostly just waiting. The exception is intensive treatments specifically designed for longer application, and that will be stated on the packaging.
Apply from mid-length to the ends, not to the scalp. The ends are the oldest part of your hair and have endured the most; that's where your damage is. Comb it through with your fingers or a wide-tooth comb so it gets everywhere. You don't need conditioner after using a mask.