How do I attach a warped panel to the frame?

You will need to put weight on the panel while the glue attaching it to the frame dries. I've had good results putting something heavy on each corner and in the middle of the panel. Very thick books or cans of house paint for example, are good weights that can flatten the panel while the glue sets. Make sure you place a piece of tracing paper or cardboard between your painting and your weights, otherwise the surface of the painting might get damaged.