Straightening your hair works best on freshly washed or day-old hair. When washing your hair, tailor your shampoo and conditioning products to your hair type. For thick, frizzy hair, use a smoothing shampoo and conditioner. For thin, flat hair, use volumizing products. Once hair is washed, dry it gently with a microfiber towel, spray it with a heat-protectant product, and then blow-dry it with a hairdryer. This will help to pre-straighten your hair, so you will have less work to do with the straightening iron.  Dry the roots of your hair with the hairdryer first, then work through the lengths of your hair. Turn your head upside down while drying your hair to add more volume. Use a paddle brush to work through tangles and hold hair taut as you dry. Investing in a good heat protection spray which you apply every time you straighten is one of the best things you can do for your hair, without forgoing straightening altogether. The heat protection spray locks in moisture and prevents hair from becoming dry and frazzled. Apply the heat protection spray evenly over dry hair, holding the can about six inches away from your head.  Don't apply the heat protection spray to your roots, as this can cause them to look greasy. Heat protection sprays can be found at most drug stores and hair salons. It may be worth investing in a brand name product which will do a better job of protecting your hair. Try asking your hair stylist for recommendations. Most good quality hair straighteners give you the option to adjust the temperature of the irons. Using the right temperature for your hair will ensure you get the best results and will prevent you from doing unnecessary damage to your hair. The temperature your straightener should be set to will depend on your hair type:  Fine or damaged hair should use a temperature of between 200 and 300 °F (93-149 °C). Hair of average thickness which is in pretty good condition should use a temperature of between 300 and 350 °F (149-177 °C). Very thick or coarse, frizzy hair should use a...

TL;DR in Vietnamese:
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