Is that you're house herds
Usually just break the walls up, centre with the face then two angled bits. Always prefer to work with hard angles if possible.
if your plastering other areas the room aswell mate use that are as somewhere to spread any raggles you have left from your set so like knock up a guage throw on a wall or so what ever your comfortable with then just try to get one or 2 of the 3 areas in the pic spread with any excess plaster you have left (not the slops you trowel off the wall though)
when you have a bay window like yous ,use a spirt level on the uprights ,when you render mate
Then pin 2 nails top and bottom in the angle and rule of them