1. Step 1, prepare black dye and 50g salt, then mix them together and put them into a pot.

2. Step 2, pour a proper amount of boiling water into the basin and stir to dissolve the dye and salt in the dye solution.

3. Step 3: Immerse the black sweatpants that need to be dyed into the dye solution. After stirring, the pants were immersed in the dye solution for about two days, so that the pants were completely colored.

Step 4: Wash the dyed sweatpants with clean water and dry them directly. Be careful not to wring them out or dry them.

5. Finally restore the black sweatpants with red knees.