A haircare tool that looks like a pair of barbecue tongs. There are two bits of metal on the inside of the 'tongs', and when you plug the straightener in, flick 'On', and select how hot you want it to go, the bits of metal will become extremely hot. DO NOT be a dumbass and grab the wrong side (I've never done that - ahem). Brush your hair out, add some heat protection balm or spray if you don't want your hair to fry, and then select a half-inch amount of hair, and clamp the two sides of the straightener together, running down the hair. Do this a few times and the hair will be left looking straight. The hotter the setting, the straigher your hair will be, but it will also be more damaging. Hair straightening is easier on thin hair than thick hair.
When the straightening is finished, mist your hair with hair spray, gloss sheen, a/o other styling products. If it's a misty or humid day outside your hair will instantly puff up and frizz as soon as you step outside -- this is why you need to make sure you're straightening your hair on a day that is pretty dry and sunny.
Your straight hair will last until you take a shower and wash your hair. If you're lucky, the hair will be left in the exact same condition as you started out with. If you're not so lucky, your hair will be left looking like you stuck a finger in an electric socket until you go to the hairdresser and get a deep conditioning treatment.
The best hair straighteners to use are the ceramic ones. The ionic ones are good too, and let's not forget the mother of all hair straightners, CHI.