In fact, assembling a PC is much easier than you'd imagine. No special documentation is absolutely necessary if you have enough skill to plug your VCR into your stereo. The easy part is screwing everything together. Basically, the motherboard should come with a manual that describes how you plug everything into it. Older motherboards had lots of problems with large hard drives and didn't come with serial, parallel, and IDE connections.
New boards have all that stuff built in.
All you have to do is plug a couple cables in and you can get it running. Since you already have a PC, try taking it apart or upgrading it. Check out our recommendations for the best parts in our System Builder section. There's lots of information to go on.