Fill a sink or bowl with room temperature tap water. Lay the flower stems in the water so the stems are fully submerged, but not the blossoms.
Cut at least 1 inch off the bottom of each stem at a 45-degree angle. The vase you plan to use determines the amount to cut. Remove any foliage along the stem that will fall below the rim of the vase.
Pour the cut flower food into a vase of your choice. Fill the vase with room-temperature tap water, leaving about 2 inches at the top of the vase. Stir the water in the vase until all cut flower food is dissolved.
Arrange the flowers in the vase and place in a location that is away from the sun or any heater vents. Lily of the Valley will last longest in temperature of 50 to 60 degrees.