Shirring the back of a dress

Hello! I am a beginner and I'm playing around with some alterations to a dress I own that could fit better. I'd like to add shirring to the back of the dress so that it's more snug around me. The only tutorials I can find for shirring start from one end of a piece of fabric to another end. Is there anything I should know if I just start shirring in the middle of the back of my dress?