I would match the pattern in the centre and finish the two pieces in the corner created by the chimney breast and wall.
Papering round the edges of a chimney breast can be very tricky and, in my view, requires experience to get a good finish. I feel paper which requires you to paste the wall will make it harder. I find working with damp, pliable paper much easier.
My terraced cottage is 180 years old. We only have two papered bedrooms these days. I've papered in and out of all sorts of corners and recesses. It's not easy and with no experience you may struggle to get a good result.
Do use a plumb line!!