You can follow these instructions:
Hinge down onto the wedge, pushing your bum back as you lower yourself down
Position the front of your sitz bones on the wedge
Reach under to pull your buttocks back as needed
Your spine should feel stacked with your sitzbones on the cushion
Look for that sweet spot where your back is neither swayed nor round
Check that your back moves with your breath
Roll open your shoulder, anchor your ribs, align your neck
If you wish a visual demonstration, Esther demonstrates this on this page: https://gokhalemethod.com/gokhale_wedge.
If you have the wedge already, you can scan the QR code in the packaging.