I make it even simpler. Standing in waist deep water in the kiddee pool, no snorkels, no scuba, they fill their masks with water and put them on full. Then we have a conversation. Once they can converse with a mask full of water, I tell them the mermaid joke: Why do mermaids wear Sea Shells? B shells are too small. Almost everyone snorts, clearing their mask. I can see if the mask is too loose/tight or if there are any other issues. Once they can talk, we clear masks floating on the surface. I've never had a student get frustrated using this process.