Line a baking sheet with baking parchment.
Place the egg white in a bowl and the sugar in another. Hold the flower, petal or leaf at the base and paint with egg, ensuring you cover every fold. Gently sprinkle on the sugar, again making sure every surface is covered, then shake off the excess.
If crystallising whole roses, push a fine florists’ wire through the base of the bloom, then hook the wire over a tall glass. Leave overnight in a dry, warm place. The flower will dry while hanging.
Lay petals or leaves on the lined sheet and leave overnight in a dry, warm place (an airing cupboard is ideal). They will harden in a few hours, becoming brittle.
Store in an airtight container, lined and interleaved with baking parchment. They are very fragile, so only make a couple of layers.