Talk to her. Get down on her level. If she is doing something wrong, explain what you want, and why it is better. Do this in a kind voice.
Listen to her. She needs to know you care and are listening to her needs:
Child: "I want a cherry popcicle."
Parent: "You can't have a cherry popcicle now, because it is so close to dinner. If you eat it now, you won't be hungry for dinner".
Child: "But, I want it now!"
Parent: "How about we take this cherry popcicle out of the box, just for you. I will put it on this shelf right here, and as soon as you are done with your dinner, you can get it and eat it. It will taste very good. O.k.?
Child: (Likely sad face, but closer to the popsicle) "O.k., but I want it right after dinner!"
Parent: "O.k., it's a deal, but you need to eat all your dinner".
Talk, listen, bargain where necessary. Help the child to see you are on her side, but you will work together. If course, sometimes a child does need a time out. Calm, loving, never out of control.