You can do this easily if you're doing a quick coverage of gray hair and if the shade varies only slightly from your natural color. Anything more complicated should be handled at a salon.  Rub petroleum jelly around your hairline to keep the dye from staining your skin. Set your timer per the directions on the dye package. Wash the color out immediately and all at once when the timer goes off. If you plan to visit a salon for a dye job, then do not use box dyes. They contain harsh chemicals that can damage your hair.   Deep condition your hair a couple of days before your appointment. Moisturizing your hair will help it to hold color better. Also, if you can hold off, don't shampoo on the day of your appointment.  Bring a picture. You can tell your stylist "strawberry blonde," but he or she might have a very different vision of what that means than you do. A picture clarifies what you want. Keep your hair healthy and moisturized. Dry hair loses color quickly. Protect your hair when you head outside in the sun. Wear a hat or a scarf, or put your hair into a braid or a knot. You can also use products that contain SPF.

TL;DR in Vietnamese:
Tự nhuộm tóc ở nhà. Nhuộm tóc tại tiệm làm tóc. Chăm sóc tóc nhuộm cẩn thận.