singing/scream help... how to do it properly?

um i wana know how to scream like Alesana or The Devil Wears Prada, or The Number 12 Looks Like You...

I really dont want to ruin my voice and no one in my band wants to take a chance so i decided to TRY and learn, but i'll need some tips..

I know that i have to drink lots of water and no acidic drinks and that it has to come from my diahprhagm but i dont know how to make it come from my diahgprhagm, do i just push my stomach out or what?

Please Help Me :)

Thank You