The only way I have been able to get the "wonk" out is to wet the piece(s) and pin them to something flat and then let them dry. Joann's has a corrugated board that folds up for dressmakers but works for this because it has a grid on it and you can place a panel on it and stick pins in it to line up the design. I usually don't wet the fabric with water, I use fabric finish (like spray starch but not as stiff). Line up and pin the first panel in a few places then repeat with the additional panels. I have been known to take a blow dryer to it to be able to get on with my project!