How do you make bearded dragons eat lettuce?

The short answer is - you don't ! Why would you want to make it eat a specific food ! Besides which - lettucehas no nutritional value - and should only be given as a supplement to a varied diet. Feeding a lizard exclusively on lettuce will result in health problems through lack of essential vitamins. Animals only eat food they like the taste of - you cannot make them eat something they don't like !