Preheat oven to 170 F. Lay a silpat or a piece of parchment paper on a cookie sheet.
Slice the lemons as thin as possible. Place on the silpat.
Put in the oven for 3-4 hours until top side feels completely dry. Flip the slices over. Leave in the oven another 2-3 hours until the lemon slices are completely dry, like chips. Leave to cool completely.
Using a coffee/spice grinder, grind the lemons to a powder.