This is now my favorite way to extract intense citrus flavors, especially from fragrant citrus (bergamot, other sour oranges, Makrut limes, and so on). I’ve started using this in both shrubs and [citrus]cellos. Basically, you slice the peel off citrus, painstakingly scrape off any white pith so you have thin, translucent strips of peel, then pack them in sugar for about 10 hours, periodically kneading them as the oil diffuses out of the peel and liquefies the sugar. Bergamot through the afternoon, in photos: 
And knobby Makrut limes getting the same treatment (I spent more time after the first photo removing pith before packing in sugar): 
