Hey Lian,
Sorry if this post gets long but this is an important question for me. Sorry also about the fact that I will once again use Megadeth as my example.

I finnaly got a band together (name undecided) and I have 4 songs in my head. Titles are: Carnivale, Robotics, The Reaping, Inner Chaos.

The album is either going to be named after Inner Chaos or The Reaping. Not sure yet. Anyway.

I know in my head what I want to do. I want HEAVY, AGGRESSIVE, CONTROVERSIAL. The first album that springs into my mind is Peace Sells...But Who's Buying?. That has songs like The Conjuring, Peace Sells, My Last Words, Wake up Dead, and of course: GOOD MOURNING/BLACK FRIDAY.

My problem is, that although I know what i want, I can't seem to get the riffs out of me. How can I write riffs of that specific style? Any tips?

Thanks a lot for any advice

Connor

Sorry to answer in Lian's forum.
My suggestion is first to emulate, create a composition that sounds similar. Borrow some of the riffs and change them a bit, then a little more and so on until you develop a hole new riff but the escence would be the initial riff. Once you have done this several times you won't have to "steal" more riffs.

I hope Lian don't get mad

why? dont worry mate feel free to add whatever you want here.

well Connor, what Ramiro said is a good tip you could use to free your mind. when you play someone else songs you see the composition from inside. this should be useful to understand how we can get those songs/riffes structures.
on the other hand, sometimes the problem is our muses or inspirations, you got the idea in your head (as you said) and you´re working on it hour after hour, day after day, but results are not what you want yet.
take in mind we are not music machines so, take it easy man.
you could start with a new song and then come back to complete the others.

Ok, I play with some metal heads that love thrash usually once a week... Here is how I compose.

First off, I listen to what others have done. I learn their picking style, their palm muting technique, their tremolo picking technique, determine what scales are commonly used.

Then 99% of the time the thrash song is in a minor key.

So pick something easy, like Em. Use the pedal tone of the open E string and mess around. Go fast, go slow, play chunky, play muted, alternate pick a line, throw a scale run in there.

Take a melody in your head and figure it out note for note. If you can't do that start studying intervals and what the distances between them sound like. This helps two fold: A - you can take the music from your head and put it down on guitar B: You can figure out other peoples songs very easily. Ear training is very important.

Eh, just my two cents on the subject.