The yucca flower is the official state flower of New Mexico (and probably also one of the most unfortunately named blooms ever). It's related to the lily, and generally hangs upside down like a bell. If you turn it the other way, it's more cup-shaped (and would probably benefit from being piped in a 2-piece lily nail like the tiger lily; this method will add days to the drying time, however). To pipe white yucca flowers, you'll need white stiff consistency royal icing, petal tip #104, round tip #3, a flower nail (with a 6-petal flower nail template stuck to it if you need help keeping your petals equal), parchment paper squares, a glue stick, a Styrofoam brick to rest the nail in, and a dusting pouch filled with a 50/50 blend of powdered sugar and cornstarch. Start by sticking a parchment square to the flower nail with a dab from the glue stick.
Hold the #104 tip almost flat against the surface with the wide side in the middle and the narrow side angled outward and upward. Squeeze while moving the tip in an upside-down "U" motion. When you reach the end, stop pressure and pull the tip away toward the center.
Touch your fingertips to the dusting pouch and taper the outer edges of the petal by gently pinching it.