Mix the sifted confectioner's sugar with the granulated sugar and almonds in a large mixing bowl.
In a separate mixing bowl, beat the eggs until frothy, then gradually add the lemon juice, vanilla extract, rum, and orange-flower water into the eggs.
Working first with a spoon, mix the egg mixture with the dry ingredients until a paste is formed. Knead with your hands to make sure the marzipan is a smooth paste. If over-kneaded, the marzipan will be crumbly, making it impossible to roll and mold.
After kneading the paste to the desired consistency, wrap it in wax paper, cover and leave it to stand for 24 hours. It will be easier to handle when you're ready to work with it. If you are rolling the marzipan, be sure to dust your board or counter liberally with Confectioner's sugar to prevent sticking.