Everyone probably has their own technique. Because the recesses are invariably not square, I do the following:
Put paper on the flat wall, cut horizontally where the lines of the window recess are, fold the flap into the recess (vertical bend line). Trim at window edge.
Cut vertical line through the paper on the flat wall above the recess corner and discard the piece of paper.
Matching the pattern at the line where you have just cut, cut another piece long enough to paper to the ceiling above the recess and into the recess top. Paste and attach it, folding horizontally into the recess. Trim edges at window.
Continue doing this to other side of the recess - and paper below the cill. Its less of a problem at the bottom as the cill doesn't need papering into.
At the other end of the recess corner top, repeat the process.