I'm not trying to be 'unhelpful' by not giving direct URLs to the information you want, but I think searching on Aminet would be your best choice to start looking.
I can't recommend one in particular because I haven't towered an A1200 before, and also there are some articles referring to AT or ATX design cases.
I would imagine your best course of action would be to download a load of the guides, read them all, then pick and choose the information most relevant to your situation.
I would only recommend trying something like this if you've messed around inside computers enough to be confident that you won't static the hardware to death etc. From what I remember reading up about it, the only 'tricky' bits are connecting the power supply to the motherboard, and the amount of weird cables you need to buy to do a proper job on it.
