Take a close look at the picture above.  Notice that the horizontal spar is sitting on a block of wood.  Behind the spar is a vertical, darker, piece of wood.  This is the leveling block. During construction of the wing,  you will use four leveling blocks mounted on the four corners of the wing.  You must make your own leveling blocks or maybe someone sells them, but I haven't heard about any for sale through any store. 

Not shown in this picture are the clamps and pieces of wood used to isolate the clamp from damaging  your spars.  Once your  ribs are placed on the wing spars,  you must be sure that they are absolutely straight, that's why we use these leveling blocks.   Once the wings are assembled, but not glued, you will place a level on the spars and clamp them to the vertical parts of the leveling blocks.  This will provide proper alignment for the gluing process.