You will need some scrap 2 x 2 or 1 ½ x 2 or even ¾ x 2 and about 4’ long. The 4’ width works for most tabletops and for anything smaller than four feet.
After jointing and planing, ripping these boards and cutting them all the same length. I mark the 4 sides. I the center of the length say at 2 feet. Then I mark another one all around at one foot, or ¼ of the length.
Next I decide which side I want to make the bow on and go to the jointer and taper joint it from about the center of the space between the one foot and two foot mark at about 18 inches from the end. Flip the caul and do the same taper cut from the other end. (Set jointer to take about 1/16” off)
Then I taper joint again starting at the 12” mark and off the end, flipping it around and doing the other side.
Now the last part, I taper joint again at about the 6” mark from the end and off the end, flip and do the other side or the caul. Now you have a rough curve on one side of the caul, but it has flat spots. To fair out the curve you could use a hand plane, but since I have an edge sander I just sand the bow into a nice curve on the sander.
This isn’t a perfect system and there are more accurate ways to achieve this, but for me the time saved by this method is worth the irregularities and I have about 50 of these that I made this way and they have served me well over the years.
Cauls can get lots of glue on them. I kept the surface that touches the glue well waxed for years. Occasionally touching up the surface on the edge sander.
But about a year or two ago I cleaned them all on the edge sander making sure even the wax was sanded off. I put a run of ordinary packing tape on the bowed side.
Also mark an arrow on the side pointing to the bowed side to make sure you always know which end is up.