To prepare the scooped-out lemons, slice the tops off the lemons to create little lids. I would also suggest slicing the bottom just slightly to create a flat surface that the lemon can stand on. Then use a spoon to scoop out the inside. You could juice this (blitz it in a blender and press it through a sieve) and store in a jar in the fridge. If the insides need a bit of loosening to remove them, use a little paring knife to gently carve around the edges.