RAID was rather difficult for me to get setup on Gentoo, but mainly because of lack of documentation. Once you know what to do though, it's rather easy.
The question I have for you is what type of RAID array do you want? Linear? RAID-0? etc... If you don't know the difference, then I can eloborate for you.
This is Gentoo specific to a degree, but will get you started. Later when I have more time, and after you've answered that question, I'll try to simplify this to you needs.