Just make sure that from the side selvedge you have a right angle at the corners. Fold the long sides together so that you have a long thin strip then draw or pin right angles on the shortest edge, and cut straight across.
Alternatively, if it's a plain woven cotton fabric and the weave is "square", you can pull out a single thread right across the curtain from that starting point - that'll give you a straight line to cut across.
There is nothing to stop you ironing a straight hem edge first, and stitching above it afterwards, to make sure that you are sewing in a straight line.