I keep following Aggie whenever she brings up ironing. Sorry Aggie. But it should be explained that ironing relieves the tight curl relaxing the paper back pretty near its original state. After ironing, place the prints under the stack of books. I use three books of medium thickness, which isn't too many. After only a day or two, the prints are nicely flattened.

Using this method, I have come to like single weight paper better than double weight. There are other reasons too, like handling the wet paper in the darkroom. But just on the merit of flattening, single weight is easier to work with than double weight. I know this is contrary to the current mainstream thought.