For large areas like a door, I would use a roller. I keep a spray bottle with water to help the paint smooth out with the roller.
Lynne...Clean the door well, let it dry and then paint as Anita described. Good luck
Here's a DIY for you! https://www.hometalk.com/8919701/sunny-yellow-chalk-painted-door
It might be helpful https://bushraslifestyle.com/how-to-chalk-paint-furniture-plus-chalk-paint-recipe/
Otherwise I have a step by step guide on how to paint a door as well.. Here you go https://bushraslifestyle.com/how-to-paint-doors-and-trim/
Let me know if you have question please, good luck!
Here are my tips for chalk paint: https://www.aratariathome.com/blog/chalk-paint-tips