The T/E is glued to the ribs next, but first the back of the ribs need to be trued up so they are all even. I use a lenght of aluminum angle and place it against the ribs. If the ribs are not even, it's very obvious and the offending ribs can easily be sanded. I've never built a wing where all of the ribs lined up perfectly. You can see this very clearly in the before and after pics of the root rib. I also use a trued up sanding block to make sure the back of the ribs are verticle.