To get into a good defensive back stance you should lean forward, have a slight bend at the waist and knees, bring his outside foot forward, a heel-toe relationship, until it lines up directly under the chin and his body weight should be on his front foot.
You must start in a stance in which you are aligned either outside or inside of the wideout. Your depth must be determined by the type of coverage you are required to play.
From this position you can then easily get into your back pedal. However, before pushing off his front foot, the DB should take his initial read to allow him to read run or pass.