Determine the amount of dye to use
You need to find out how much dye is required to refresh your black denim and for this you need to measure the weight of your denim. Usually around one pound of fabric needs one box of powdered dye. In case you are using liquid dye, you will require only half bottle for one pound. However, you are applying black so you are required to double the amount which is normally used. You should apply two boxes of powdered dye or one whole bottle of liquid dye in order to get the colour you desire.
You need to wash your pair of denim jeans and then leave it to dry. Remember that you should not dry the jeans as you are required to use the denim when it is damped.
Use the dye
Next you need to take a bucket and fill it with hot water. Add the powder dye in a measuring cup with two cups of hot water. Later pour it in the bucket. In case you are using liquid dye, pour it into the bucket and then stir it.
After dissolving the dye, you need to add 1 cup of salt in the back. Add the fabric in it and then stir it. Make sure the dye completely soak into the fabric.
Rinse and wash
Next rinse the fabric in warm water and then in cold water. Later put the fabric in machine with warm water with a light detergent. In the end rinse it again in cold water.