This is a common question. If the wall is a little uneven, Bellseal Fix’n’Seal is flexible enough to be used.
If the wall isn’t square (ie the shower or bath touches one end but there’s a gap the other, remove the bath or shower and build up the surface with plaster to fill the gap, and make good as required. Off the bath or shower back to the wall and get it as close as possible. See our video for a guide to applying it in corners.