Here's two links that I found pretty easily. First one is dating a bit but it's basically the same thing. It has also great pictures :
This one seems fine too but no pics some annoying ads :
It's really not that complicated, just need some patience, reading and if possible, access to another computer with internet to seek for some help if needed!!
Also, if you buy new, your motherboard and processor will come with manuals that tell you how to do it
The most important manual is the motherboard's manual since jumpers, connectors, etc often change between model. If you don't have the original motherboard, you will be able to download it from the manufacturer website, even if it's an old model.
And last thing, make sure you install the CPU fan correctly, could be fatal! And double check the power supply voltage selector at the back (110V for N.A. or 220V for Europe --- not sure myself, DOUBLE CHECK!) That could be fatal too.