1. Squeeze the lemons into a glass measuring cup.  Add additional lemons or limes to reach the 1 cup mark on the measuring cup.  
2. Stir in the allulose and whisk until the allulose fully dissolves.  Pour the mixture into ice cube trays or into a container.  Place the mixture into the freezer.
3. When ready to use, add 2 tbsp of the lemon concentrate to 1-2 cups of water.  Add additional water or lemon concentrate to suit your tastes.

1 ice cube to 1 cup water