Yeah dude just keep messing with how you do it till you get it. You don't really need any tutorials (although benoits metal rhythm tutorials are amazing and good for much much more than just palm muting). I don't remember how i learned I think I just did it one day and then after that it's cake.

Kinda like Pinch harmonics I guess it was really hard to get it to sound right but now I don't even need to think about it I just do it when it fits. Just keep playing and things just become natural.

I always say use a metronome to increase speed, there's no reason really not to. Yes it can be boring I guess but if all your doing is sitting there playing the same thing but without the metronome then why not also practice beat sub dividing and timing which are all important in fast playing. The best thing to do (and one of the most fun imo) is to just sit there and make noise and see what kinda crazy things you can get your guitar to do, I'm sure that if you just sit there and play you'll figure it out in no time.