When building on a board, over protected plans...[ul][*] I lay down the lower spar(s) and trailing edge usinng a few ribs to get them spaced right and[*]Pin over the spars (criss-cross above them, not through them) and through the traling edge stock.[*]Then, I apply glue to each spar where each rib joins it and[*]Take each rib and put glue in the lower sparspar slot(s) and... [ul][*]Position the rib squarish over its spot on the plans,[*]Align it vertically (for which I have some little square bits of metl, but a triangle from a ctudent drawing kit at the dollar store is really good, or anything small and really square) and[*]either pin it square or wait for the upper spar to hold it[/ul][*]For a small wing (which doesn't have too many ribs so I can adjust them before the glue dries, I glue the upper spar slots, insert the upper spar(s), and come back with the square to get each rib nice and verticle and the spar holds them aligned.[*]For a larger wing, I just pin the ribs down to the board as I go since the glue may be to stiff to do much repositioning before I'd get an upper spar positioned to hold them.[*]Also, neatness counts because unnets stuff has weight after all that, I go over each joint with a rounded stick to leave a neat glue fillet that bridges the whole length of every joint.[/ul]
There are ways to be more sure that things are perfectly square, but it turns out that you can forgive yourself for some small imperfections as long as you tried not have them and they aren't too bad.

Notice, I can adjust and tweak things really percisely because I use wood glue... if you really like CA you can do about the same thing but you have to have each rib alligned well before the it gets its first drop of glue.