Figure out if the scene requires this type of scream. You don't want to scream like a girl, if you're the hero in the play and about to go to battle. Most times, an actor will scream like a girl to bring comic relief to the scene.
Open your mouth wide. To scream like a girl, you need to open your mouth and throat wide to make the necessary sound.
Use a higher pitch than usual. When you scream like a girl, your yell needs to sound much higher. Test out different pitch levels in order to sound authentic.
Make the screaming sound from the back of your throat. It should not come from deep within your chest like a more masculine sounding scream.
Keep the scream short. The longer you try to scream like a girl, the quicker you'll lose authenticity. Limit the screaming sound to a few short seconds. If you have lines to give after the scream, silently clear out your throat before you speak.