Yeah it is easier to have someone show you visually, somethings that stumped me made sense once I was shown, or I had a chance to talk to someone personally about it. Even writting posts is kind of hard.
Another thing that is good in writting heavy stuff is to use your blues scales. One example, Black Sabbath, although their music now is kinda boring to more contemporary metal. The idea of utilizing blues scales can add to a metal style of playing. Especially over a major chord, the blues note adds a dark doomier sound. Here's just a simple example.
chord-C#m-----C#M-----C#m-----F#m---------
scale-C#min---C#Mblue-C#min---F#min-blues-
d:----------------*-----------------------
a:----4-6-7-6-4-7-8-7-4-6-7-6-4-33-0-3----
e:----------------------------------------
* Listen how the F makes that particular part darker than if you were to play F# instead. Which would make it major sounding. The major blues note (b3) is a good way to turn what would be a major happy chord into a darker sounding chord. This even works with heavy palm muting sequences. Say you write a song in C# minor, and you didn't detune your guitar. It's still in E. Well E major is the relative key of C# minor, so you could use an open E for palm muting and apply the major blues note like this:
g:----*-blues note----------------
d:--6-5-4-------------------------
a:--6-5-4-2-2-2-2-2-2-2-2---------
e:--4-3-2-0-0-0-0-0-0-0-0-..cont.-
pm------------|
So instead of an E major sounding sequence, it's sounds alot more minor. A good heavy and simple riff, and you'll probably seen something like it. It's a great way to metalize a major chord. Works great if your coming from C# minor chord (relative minor) too. Just play C# then G#, G, F#... C#minor is a great key for metal. I think of the C# as the "devils note", because it's got a strong sound to it with a darker edge. Hope that helps.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.