This is a hole I dug to expose the clay- mineral layer in the soil. Normally the top layer is made from organic material. and a little way down is the mineral layer consisting of very little organic material, and mostly a mixture of clay and sand. Sometimes the mineral layer is absent and instead have snad and gravel.
If you find a mineral layer, like the reddish layer in the photo, you have the perfect soil for the bees to pack their clay back to the nest and use it for plugging up their nesting tunnels.
If there is only sand underneath the organic soil, you may have to buy or find some local clay and smear it on the wall of the hole that you have dug. If this is so, watch the video and see how mixing clay is just like mixing flour with water- a little of water at the time.
Video of preparing clay for mason bees.