The easiest way to build a flat bottom wing is on a flat building board. My building board is a cheap hollow core door from Home Depot. You can use anything that stays flat and holds pins. In the following photos, I’m building an RCM Basic Bipe.
Balsa sheets frequently are warped. When you build a 20 or 40 size plane, you will usually have to split the sheeting to make it narrower. If you use a yard stick as a cutting guide you will create a straight edge. Find the straight edge and arrange your sheeting so the straight edge will be on the edge of the wing, rather than marking the boundary of the wing with a warped edge.
This is a warped edge.
Notice that this plan shows the top wing on the left and the bottom wing on the right, so you have to mark the rib locations on at least half of the sheeting. You can build with no plan at all if you mark all of your rib locations and draw a couple of parallel lines and a perpendicular line on your table. After you mark your rib locations pin the sheeting down on the building board. (As you can see in the photo, I’ve actually drawn the rib locations on the bottom spar.)
Glue the spar to the bottom sheeting, then add the ribs. Use a square to make sure the ribs are vertical.
After you place all of your ribs and glue them, add the top spar and the top sheeting. For the top leading edge sheeting, choose wood that will easily bend without cracking. If you don’t have any, you can get the wood to bend more easily by wetting it. Wet only the outside, then stick the sheeting onto the top spar and the ribs with medium CA glue. Before you add the top sheeting, pull the pins from the bottom sheeting, and hold the wing flat on the table with your hands while adding the sheeting. Unfortunately it’s hard to take photos of this step because I can’t use my hands for the camera and installing sheeting at the same time.
When you have the top sheeting, bottom sheeting, and leading edge in place, your wing can still twist. When you add shear webs, you will make your wing stiff and untwistable. Set it flat on the table and hold it in place with cans of beans, or some other heavy object. Place the shear webs one by one, using medium CA glue.
The leading edge, the top and bottom sheeting, and the shear webs form a structure known as a D-tube. When all parts of this tube are complete, the wing will no longer twist, and it is safe to pick it up and add parts to it without it being on a flat table. But I like to keep the wing flat on the table while adding trailing edge sheeting as well, just to make sure. Find the straight edge of the trailing edge sheeting and put it at the trailing edge of the wing, to make sure the wing is straight. If one side is warped, put it toward the inside where it doesn’t make any difference.
Now everything is pretty much locked into shape, and you don’t have to worry so much about it staying flat.