You need to have it on a lot tighter than you think when you first put it on.

Hold the pocket over your hips and then with one hand behind your back hold both pieces TIGHT in one hand.
Don't let go of that tension in the hand behind your back while you pull each piece over the opposite shoulders with the other hand and down the front and under the pocket section.
You can then make them tighter if you need before wrapping the exess around you and tying it up.

Before you pick up bub then pull at the front bits to loosen them a tad before getting bub in. The fabric will loosen up with body heat like a pair of jeans.