Ted Blachly replies: When making a top, use lumber that has been acclimated to the shop for as long as possible. Flattening a single board is a basic woodworking skill but requires sharp eyes and sharp hand planes. Use straight edges and winding sticks to check for twist.
Once a side is flat you can proceed to the planer to mill the other side parallel. Re-check for twist as you approach desired thickness. Joint one edge square to the face and run it against the fence of the table saw to rip the other edge square. If you can do that, you are halfway there.
I have a set of perfectly straight long and heavy maple cauls that I use for big tops. I attach them to my horses and make sure they are parallel by shimming the feet of the horses – check in the same way you would use winding sticks. I wax the edges of the cauls to avoid glue sticking and lay my top boards on perpendicular to the cauls. Check the joints. If the edges are square and the set can be drawn up dry with light clamp pressure, I’m ready to glue.
When placing the clamps, I work “west to east” alternating one on top and one on bottom. I try to center the bar clamp screw in the center of the edge rather than put the bar right on the boards. Excessive clamp pressure can cause distortion in the top. If the top is laying flat on the cauls when I’m done tightening the clamps, then it is flat.
Another thing I do routinely when building a project that has a substantial top is after I’m done making the top (and it is flat) I’ll screw some temporary heavy cauls to the bottom to maintain “flat” while I work on other parts of the project.