Cerinthe is, without a doubt, one of my favorite fillers of all time. The colors are stunning and it's one of my earliest blooming fillers here in North Texas. It also sprouts easily from seed, no complicated seed tray setup or greenhouse necessary. The old-timey name is Pride of Gilbraltar - a small peninsular near the Mediterranean sea)
I plant two batches of seed, the first in the fall around October when temperatures are around 65 degrees. The second planting is during our typical January warm spell (now). I keep the soil moist until the seed sprouts, within 3-5 days.
For my October plantings, I sometimes cover the plants with agribon (a row cover) if the temperatures dip below 25. You don't need to do this, but the cold may blacken the leaves and you'll need to trim them off. The plant won't die and as soon as it warms, new green shoots will grow up from the plant. It will, however, be more robust and productive if you can cover it.
Around early to mid March, you'll see delicate little purple flowers begin to form. This is the perfect time to harvest. If you wait too long (a week or two) those purple little flowers will turn into seed pods and will shed everywhere :) If you'd like to leave a couple stems uncut, it will easily reseed itself for next year.
Bees and pollinators LOVE Cerinthe and will be happily buzzing around your plants when the day starts to warm, so harvesting is best done in the early morning before the bees arrive. Strip the lower 2/3rds of leaves on the stem so it won't wilt and put straight into water in a clean bucket or vase and then let sit in a cool room for several hours to condition.
Trick - if you wrap construction paper into a cone (staple at the base) and put your stems in the cone, supported upright for a couple hours in water, it will help keep them from flopping over in the vase. Cerinthe has a beautiful natural arch to the stems and that's part of their charm!