kindof stiffing your gut in such a way that if you were to go UGHH like you were furstrated and it wouldn't hurt, but not hard enough that it makes your stomach uncomfortable. Take like a Cat's hissssssss and try and mix that with a voice or tone using the UGH method to help it from not hurting. PAIN MEANS YOU ARE NOT DOING IT RIGHT. Experiment with your mouth shape to learn to change pitch , for death metal grunts you will want something uhhhh, ooooo, ohhhhhh, owwww. type things.
sorry for this crappy and not very in-depth guide, but just try and give it a shot