well, i recently have been working on my first metal song (which is currently on hold due to my comp frying itself... literally), so im by no means an expert, and i dont know how much you know of guitars or music in general, but ill tell you what i have learned.
first off, find some good clean guitar sample sets, with sustained/muted/harmonics. i found a really nice one called the 'damant spectrum guitar', i dont remember where i got it from tho, try searching google.
next, reserve 6 channels for the guitar, one for each string and label the channels with the string's low note (this makes it pretty easy to do strums, and makes it easier to think of how the part would actually be played).
next, get a good distortion vst and apply it (maybe a flanger, chorus, compressor, reverb or eq also, but the distortion is the most importaint), best one i have found for this is mda combo, its perfect for the heavy metal feel. i would suggest experimenting with the rez filters too, especially for the lead (maybe varying them as a guitarist would do with a pedal).
also, when you are writing the parts, use the muted and harmonic sets in places where they sound good (or if you know about guitars, where they would actually be). like, try putting in a couple short muted notes at the end of one pattern before a long sustained note at the beginning of the next.
so yah, thats about all i can think of, hope some of it helps.