What I do now is:
Dry it at a hanger or flat on a glass plate.
(Before I taped it to the glass, but due to the shrinking in the length region of the paper, it looks different than the one dried without tape.)
Next I flat it under a press at 85 degrees C.
Next step is to get an archival piece of thick museum carton.
put a piece of hot press tissue at the back.
position it onto the carton and press it for 1.5 min at 85 degrees.
Next i take it out and put it between two heavy wood boards to let it drop temperature so that the glue will set itself in this position.
If you let it drop temp. without pressure the carton, it will get an angle, because one side is glued and the other side not.