In a saucepan, add water and sugar on medium-low heat until the sugar is fully dissolved.
Add lemon slices and simmer for about 15 minutes or until the lemon slices become slightly translucent (try to keep the lemon slices in one layer).
Turn off the heat and place the lemon slices on a single layer on a wire rack.
Let the lemon slices cool down for about 1-2 hours. Then cover them and put them in the fridge.
Notes
If your syrup becomes too thick while simmering, feel free to add more water to thin it out.
You need to slice the lemon as thin as possible. You can either use a mandoline or a sharp knife. If the lemon slices are too thick, they will take too much time to cook.