Set oven on the lowest setting available. A heat setting ranging from 150F (65C) to no more than 200F (93C) degrees is highly recommended.
Place all of the flower heads onto the baking sheet, spreading them out so they do not touch. If using baking racks, place them onto the baking sheet first, and then fill them with the flower heads.
Set the time for at least 1 hour, but up to 1.5 hours if preserving a lot of flowers on multiple trays. It may take up to 2 hours to dry the flower heads if they large or thick.
Check on the flowers about every 30 minutes to avoid over-drying them and causing scorching.
Remove the flowers from the oven, and allow them to air dry to room temperature before storing in a container with an airtight lid.