Hi
I haven't seen a comprehensive guide anywhere before. But when I did mine a few years ago, I just carefully removed the originals and took a few photos. There are some quite savage sharp tags of metal on the backs that you need to watch out for, as they can tear seat covers and skin!
The tags need bending back carefully to get the covers off, then bending back to hold the new covers on.
Also it helped when re-fitting the cover to warm them up so they are more subtle (mine were vinyl TMI ones), it was winter so I put mine on the radiator, I think in California they just leave them in the sun! I have heard of a tumble dryer been used too.... It was quite a wrestle to get them on, and to get the corners to look good.
I was recommended the foam pads by the reseller when I bought them, but with hindsight I would go for the coir ones, for comfort and best shape, the foam is a bit soft.
Hope this helps!