The formula to make an Orange or other Citrus liqueur, such as lemons, bergamot, mandarin, bitter oranges, pomelo, etc is very simple. You take the peel or zest and steep it in alcohol to extract the essence of the fruit. Then you make a simple syrup with sugar and combine both together.
Bitter Orange
Kitromilaki or Nerantzi Glyko, is a fruit preserve made with Seville Oranges when they are small and the seeds have not yet developed inside.