How do you build a balsa bridge, step by step?

Short answer: read the rules, draw the design full size, cut and sort the sticks, glue each flat truss on top of the plan, join the trusses with cross-bracing, then weigh and test.

1. Read the rulebook and make a one-page sheet of limits. See reading your rulebook.
2. Choose a design. Pick a simple truss that fits the span and clear space. See truss basics.
3. Draw it full size on paper, with each member's length and cut angles marked, and make a cutting list.
4. Sort your sticks by weight. See choosing sticks.
5. Set up the board. Tape the plan down, cover it with wax paper, and gather pins.
6. Cut the members, a little long, and sand each to fit. See cutting joints.
7. Build one flat truss at a time. Pin sticks over the plan, dry-fit, then glue. See gluing.
8. Make the second truss to match the first.
9. Join the trusses with cross-members, keeping them square and parallel.
10. Add the deck or load attachment the rules call for.
11. Weigh and measure against every limit.
12. Wait for the glue to cure, then test a practice bridge and improve the next one.

Build a practice bridge first if there is time. The first one teaches the most.