Don't rush it, flat panels don't happen by accident, see how we do it in the WOOD Shop.
Board with metal square ruler
Use a framing square or othernstraightedge to check the panelnfor flatness.

Checking a panel for flatness

Check the assembly for flatness with a straightedge as shown in the photo. Once you have everything tweaked into position, leave it alone for at least one hour. If you move it, something will almost certainly move out of adjustment. After gently removing the clamps, place the panel so there is no strain on the joints, and so air can circulate freely around it. Don't try to rush things by putting the panel in the sun--that's an open invitation to warping. When using yellow woodworker's glue, wait at least three more hours before machining the panel. With white glue, allow overnight drying.