I don't think that the talent of writing death metal has anything to do with how long you have played guitars or how weel you know the scales and stuff. I have friends who have played guitar for ten years, but still they can't write even decent riffs. You just have to understand the structures of the riffs. Kalmah's riffs are pretty easy understand, since most of them are pretty simple. Beutifully flowing göteborg-stylish melodies are the hardest ones to understand, since the structures are sometimes very difficult to understand. That's why I like In Flames so much.
I don't know anything about the guitars, the notes, the scales or the "theory", I just play the goddamn thing. I have played guitar for about three years and I have over 600 riffs written down. These riffs include everything from thrash-metal riffs to göteborg-melodies. Almost half of them are very complex and it's getting better all the time. I'm not the right person to rate them, though...but I only write down the best riffs that I come up with. My earlier riffs are shitty, because it takes a lot of practice to learn to understand the structures of the riffs. I have also come up with some solos, but it's sometimes really hard to put your thoughts on the guitar, and that's why sometimes I just can't write some riffs down.
My advice is: Just listen to the melodic death metal and try to understand how the things work. Mess around with guitar. It's hard to explain it. Maybe it just requires some special skills
But if you can't learn to understand these things, I suggest that you don't even try to write down any riffs...forced riffs don't sound good. And how do I provoke inspiration? Sometimes I can come up with like five riffs when I'm taking a walk with dog...sometimes I get the inspiration by listening to some very good cd... and sometimes I just decide "ok, now I'm going to come up with a goddamn excellent acoustic riff" and then I just start playing with the acoustic guitar until I find what I'm looking for.
If you want to learn an easy way to compose simplistic Kalmah-stylish riffs, just mess around with the 2nd, 3rd, 5th, 7th and 8th frets of the e-string and the open e-string. For example
0-2-0-3-0-5-0-7-0-5-0-3-0-5-3h5p3-2
And voila, there you have it. It's not even a decent riff, but it's something Kalmah would play (if they were crappy).
I hope you understood even something about this post, since it's very difficult to explain these things I you are Finnish and don't know much about the guitars...