Not starting your down swing properly is a common problem, especially for people who pull or slice their shots.
This excellent video, by MeAndMyGolf, explains the proper sequence of how to start your down swing, and is a variation of other blog posts such as “Box Before Ball” and “Lead with the Lower Body“.
The key is after you get to the top of your back swing, shift your weight into your front foot, turn your hips, and then the shoulders and arms will follow.
Check out other videos on Youtube by MeAndMyGolf for other great tips.