Next, peel them and place the peels in a non-reactive bowl or jar. Try to keep the peels as free from pith (the white fibrous layer under the skin but before the pulp) as much as possible.
Gently mix in an equal amount of sugar, by weight, to the peels. The sugar will quickly become sandy and stick to the peels. (optional: also gently muddle the peel and sugar mixture after about 10 minutes on the counter) Let the mixture sit for 24 hours at room temperature.
The sugared oil left over is your oleo-saccharum. Strain the syrup into a sealed container and refrigerate for up to a week.
Notes
To get 1 cup (60-65g) of peels you will need roughly: 6 lemons; 7-9 limes; or 3 grapefruits. Unfortunately, I can’t give a good estimate for oranges. They vary so much in size, depending on varietal, that it’s really impossible to say. Your best bet is to compare the oranges you buy to the size of the other fruit listed.
Play around with ingredient ratios! I use a 1:1 ratio by weight, as is traditional, but you can add much more sugar for a gently-citrusy syrup.
Oleo-saccharum yield also varies by fruit. Lemon peels make the most with between 1 1/2-2 ounces (45-60ml). Grapefruit and/or orange peels came in second place with 1-1 1/2 ounces (30-45ml) And, last but not least, lime peels only create about 1/2-1 ounce (15-30ml) of sugared oil.