The point of the double push is to go on your outside edge and carve the wheels on your to skate into the ground to make a stronger push.
To start off with the double push remember to stay low and bend your knees so you have range of motion. Now you can get going, what you do first is push out to the side as you shift to your outside edge and start your carve as you push.
While you’re on your outside edge bring the foot that you pushed out to the side around almost behind you other leg, then bring your skate through to the side of your other skate and step out slightly and repeat on other leg.