I've done one and it wasn't to bad other than the amount of carbon build up around injector which had baked onto the head. This needs to be removed 1st to try and avoid any carbon falling down injector hole as its removed.

I scraped most of it away and used a vacuum to suck what i could up.

I think there are different types of wahser thickness so check it out 1st.

I removed the egr valve from the back of the head as i couldn't separate the metal pipe at the clamp for some unknown reason but you'll see what i mean once your in there.

My advice though is do them all at once which i regret not doing. We've done a couple of peugeot's since and always do all 4.