Basic method
Steep the botanical in hot water first (like tea), strain, then dissolve sugar 1:1 by weight. Steeping in water before adding sugar extracts flavor more cleanly than steeping in syrup.
Ideas
Cascara, hojicha, pandan, black lime, lapsang souchong, Szechuan peppercorn, toasted rice.
Shelf life
About 2 weeks refrigerated. A small splash of neutral spirit extends shelf life in full-proof drinks.

