You can restore the right angles by pulling the fabric back to where it should be - there is usually a description and picture or diagram of how to do this in most books on sewing, but basically, one corner will be narrower than 90 degrees, so you take hold of the two edges, the selvage and the weft, and gently exert outward pressure , so widening the angle. You work away from the corner and then along the length of the cloth stretching it diagonally so as to draw the warp and weft back to 90 degrees.

It might take some time, and a couple of workings over, but I would not recomend that you start to sew before you have done this, or you might sew the wrongness into the finished kilt.

If you have someone to help you you can work more than an arm span apart, pull on the wide corners along the whole ength of the cloth, and that might get it done faster, though you do need to be careful not to damage the threads, you are only trying to adjust them relative to eachother, not actually stretch any of them.

Anne the Pleater