Having a lot of trouble learning to death growl/scream, please help?

I'm a huge fan of harsh vocals and death metal (Opeth, Necrophagist, Bloodbath, a lot more), and for a while I've been trying to learn how to growl and scream. I follow all of the steps that I've learned through other people and videos on youtube and whatnot, but I still don't know how to add distortion to my growls. When I do it, it just sounds like I'm saying "OH" loud, in a really deep voice. I feel vibration in my throat as well but its not producing any distortion to make that brutal death metal sounding growl. and if any one has any info on screaming (Dillinger Escape Plan, Meshuggah) I'd like to have a link or something to learn that, or at least an in depth explanation, because this fry stuff sounds like a loud of bull.