If the panel has any kind of a "landmark" line - an actual line or a division between two different prints or colors or something that I can go by, I draw a line on my (huge, homemade) ironing board with water-soluble marker, then use a square ruler to mark a perpendicular line at one end.
I get the panel pretty damp with Best Press and start pinning the "landmark" line on the panel to the line on the ironing board. Put a pin every inch or less on that side.
Gently pull against the pins to smooth out the panel and line up the top of the panel with the perpendicular line on the ironing board. This may be the hardest part. You might have to get the fabric a little wetter to force it to behave.
Get a ruler and measure from the pinned landmark on one side to a similar point or line on the other side of the panel and pin at that spot. Smooth and measure and pin, smooth and measure and pin.
Let the panel dry on the ironing board, then press it. It's a pain in the neck, but it's easier than trying to piece a tetrahedron! :mrgreen:
Good luck! :)