There are many ways to cleanse ones aura/energy. Some of the most used ones are cleansing by fire, cleansing by energy and cleansing by water. I will be talking about cleansing by fire and water in this post, cleansing is advised to be done before and after all spell works and it can also be used just to make your mood better.
To start with energy cleansing sit in a pose so that your back is straight and you are relaxed, the lotus position is one such post. After relaxing to an extent start breathing in a rhythm [inhale for 3 seconds and exhale for 3 seconds]
While doing this visualize pure white/blue energy (green if you want to heal) being inhaled and black/red energy being exhaled. Do this until you feel better and clean.
Cleansing by water
To do this you need a shower or bathtub. once you have one of them ready visualize the water being a pure white mass of energy and that your aura has lot of dark/black energy. then step into the shower or into the bathtub then visualize and feel the dark or bad or negative energy being dissolved in the white mass and the white and pure energy entering and becoming part of your body.