In a food processor, whiz together the ground almonds, icing sugar, and sugar. Add the lemon juice and almond extract. Whiz again for a few seconds. Add just enough of the egg to make a firm paste -- no more (you will likely have leftover egg.) Remove, and knead a bit with your hands until smooth, then use at once, or wrap in plastic wrap and store in fridge and use up within 4 days.