It’s best to do this once you are ready to start the dyeing process. Pour warm water into a medium-sized bowl. Sprinkle in the indigo powder. Wait for about ten to fifteen minutes. You will want to mix this until you create a textured sort of paste. You should now have a prepared indigo mixture. Spoon this on your hair using your hands. You should place indigo on all parts of your hair that you want to be darkened. You want to make sure the indigo stays warm and seeps into your hairs. Therefore, once you have covered your hair with indigo, you should put a plastic wrap or a shower cap on your head. You will want to leave your hair covered for about an hour. Using cool water, you should rinse out all of the paste that is in your hair.  Do not use shampoo for the next two to three days. It will take a couple of days for the indigo to oxidize. Once it does, then you will see the actual color it was supposed to produce.

TL;DR in Vietnamese:
Trộn hỗn hợp lá chàm. Phết hỗn hợp lá chàm lên tóc. Bọc tóc. Xả sạch hỗn hợp.