I don't think the facial muscles should be active when mewing. So that means the muscles around the mouth, the jaw (masseter) muscle need to be relaxed. Everything needs to be relaxed and natural as possible. 

You know how people always say, "teeth and lips together"?. Well I have found that if my teeth are touching together then my masseter is activated. So i have them slightly apart. This could be something to do with my bite though. I don't think I have a deep bite.

Also, I don't close my lips together. Why? Well, the lips are meant to be closed and touching as I think the bottom lip supports the upper lip, which gives you a nice lifted upper lip. When certain people close their lips, their upper lip comes down and the bottom lip comes up to close them. This is wrong. The top lip should not come down because this activates the muscle above the top lip. So currently.. when I bring my bottom lip up to touch my upper lip, the mentalist (chin) muscle is activated. I do not want this muscle to be active. The reason why the chin muscle is activated is because my bite needs to be deeper so the lips touch without any muscle being active.

I could be wrong with what I say but it makes sense to me.

Facial muscle activation is a sign of a weak tongue. Stop pushing your tongue as soon as you feel your facial muscles activating. That's the limit of your tongue. Work on getting it stronger with exercises such as tongue chewing.