Pin a straight edge down to the plan and then push the lower spar against it. Pin the spar in place and then remove the straight edge.

Keep the wing pinned down until the glue is COMPLETELY dry. I use weights all over the wing to hold it down and move the weight around to keep it out of the way of whatever I'm working on.

These two pages show wings under construction. Neither page is about wing construction so you'll have to decipher what you can from the photos.

Scroll to bottom of this page:

http://www.airfieldmodels.com/constant_chord_ribs.htm

On this page the wing was built upside-up then turned upside down to add the lower sheeting and cap strips.

http://www.airfieldmodels.com/tools_tips.htm