If you want to dye dark jeans with color, bleaching the area you wish to dye beforehand will yield the best results. If your jeans are light wash or white, you can skip the bleach. Start by wetting your jeans. You should wet the area you want to dye, but wetting the entire pair is fine, too. You can wet them anywhere—in the shower, with the hose, or in the sink. Lay a cloth or tarp over your workspace to protect the area from the dye. Wring out any excess water and lay your jeans flat to dry. This will take a few hours. Then, feel free to add finishing touches, like fraying or rips!  Check your dye label for any special drying instructions. Wash and dry you dip dyed jeans by themselves for the first time. After that, feel free to wash them with other clothes.
Bleach dark wash jeans before dyeing. Wet your jeans. Prepare the area. Allow your jeans to dry and style as desired.