People dye jeans for different reasons. Some dye them to revive the new look their jeans once had. Some dye their jeans to try and cover up some problem with the jeans like a stain or bleach spot. Some dye their jeans to get the exact color of jeans that they want and others dye in unique ways for cool effects.
No matter your reason for wanting to dye your jeans, there are a couple of things to keep in mind.
Dyeing is messy and the mess is pretty much permanent, it is dye after all. That's why I like the bucket method of dyeing.
When you dye jeans, everything gets dyed including the stitching. Stitching on jeans is generally yellow, although it does come in other colors. You may or may not have noticed the stitching on your jeans, but it is really a major part of the look of the jeans.
The dye will look darker when wet and until after it has been washed the first time. The final outcome will be lighter.
If you have never dyed jeans before, I wouldn't recommend starting with your favorite pair. If you have an old pair to test your process on, you can make adjustments before dyeing a pair you really care about.
Here are the steps per the RIT Dye website, other dyes may need to be done differently.
Fill sink or bucket with enough hot water for fabric to move freely. Remember: Use 1 package of RIT powder or 1/2 bottle of RIT liquid dye and 3 gallons of hot water for each pound of fabric. Pre-dissolve powder in 2 cups of hot water. Add pre-dissolved powder or liquid to dye bath. Stir to mix.
Wet fabric in hot water. Uncrumple and add to dye bath.
Stir constantly (back and forth, up and down) for 10 to 30 minutes.
Rinse in warm water, then gradually cooler water until water runs clear. Wash item in warm water with mild detergent and rinse thoroughly in cool water.
Dry item in dryer or air dry.
Clean sink or bucket immediately with chlorine bleach or scrub with chlorine-based powder, liquid, or gel cleanser.