After cladding the walls and ceiling (see post) I went and did the side door.
Note: This post is backdated. Added 15th May 2015
1. Fit cladding to door
I screwed the cladding straight into the metal of the door making sure that each piece lines up with the cladding on the wall so the panels continue nicely when the door is shut.
I used a utility knife and sandpaper to fit the edge profile of the door, and to cut around things like the door handle, lever and window.
2. Fit cladding around window
I found a neat trick for fitting the pieces around the corners of the windows. I built up around the corner with bits of masking tape and then removed the tape as one piece to use as a stencil to cut the curve for the piece of wood in the corner.