I had washed mine before I knew it should not be.
I asked the LQS where I was taking the course, and they said, not to worry ... press it well, as well as spray starch it well.
The starch put the sizing back in and allows the layers to slide for you as you are getting the layers matched up.
I had NO problems!!
As always ... I am glad it is pre-washed ... no fretting about shrinkage or colours running later.