A decent graphic or video would demonstrate better but I couldn't find one so...
If the window is centred I would start either side of the window with a full strip. If the window reveals are not too deep (more than half the width of your wallpaper) I would mark the line for your first sheet so that when a snip is made at window board and top reveal the paper can be brushed around the side reveal up to the window and trimmed along the window then trimmed under the window board.
Go nice and slow when brushing/smoothing around the reveal into the window making sure to keep your brush hard on the paper as it goes around the edge (to avoid a corner of air).
If the reveals are quite deep mark your first line half to three quarter width of your roll away from the reveal edge, proceed as above, then piece back into the window.
If the window is not centred I would start on the side with the longest (widest) bit of wall.
Depending on the size of your window and paper pattern you may be able to butt join pieces at the top/bottom of the window and continue with a full length the other side, likely having to piece a small strip to complete the reveal.
If there is no pattern you can hang a full sheet on the other side of the window then fit pieces over and under the window allowing them to overlap. Run a very sharp knife in the middle of the overlap. Remove the waste from the top and underneath of the overlap to give a flat joint. You can also do this with a patterned overlap if that makes it easier to get full strips either side of the window.
It might help to first hang a full sheet on the other side of your first line as it can be easier to keep the window sheet plump butting to another sheet than working to a line only.
It can be fiddly and time-consuming so you may not want to paste too far ahead of yourself.
Good luck (if you've not done it already!).
Meant to say - if the paint is sound there shouldn't be any problem papering over it.
