if you want to learn to scream drink plenty of water. i know everyone says that but trust me its absofuckinlutely neccessary. also scream from your stomach not throat. youll eventually understand how to do that on your own. and depending on what kind of scream you want you have to form your mout in diferent way. example

high scream – tounge low mouth open
low scream- mouth a little bit open with tongue low.

wellll hope i could help a bit