What is the best way to make balsa wood flexible for construction of a bridge?

A big factor to the "Difficulty" will be the size, thickness and lenght of the wood to be bent.

Steaming is quicker to soften the wood but can difficult to do on large pieces. A steaming box is made to hold the wood while steam is piped into one end and allowed to exit the other. An advantage to steaming is that the wood dries quicker than if soaked in water and steaming is more effective on thicker pieces.

Weather using a steaming box or soaking the size of box or pan must be large enough to hold the longest piece to be softened.

HINT: For soaking longer pieces, look into getting a water tray for soaking 36 inch wide preglued wall paper. These are under $5 and are available where wall paper is sold. If yo need to soak a longer piece you can use a section of rain gutter with end caps to seal the ends.

HINT2: The effects of soaking can be soeed up by using hot water AND added a few drops of dish soap to cut surface tension of the water. (not a lot of soap you don't want to see a lot of suds) Rinsing in clean water then bend and dry as usual.

Soaking in water is easy to do and safer (no high temp steam) it does take longer to soften the wood and to allow to dry.

In both cases(soak/steam) the softened wood is bent and held in position while it cools and dries. The wood is clamped against a form or pressed between two forms to hold its shape. Once fully dry the wood can be glued and will hold its shape.

HINT3: You can speed the fabrication process by using Gorilla Glue which uses water as a catalyst, so the wood need not be dried fully to glue together

HINT4: When gluing laminated sections place wax paper under the form and work table to prevent glueing your work piece to these items.

Bending Balsa Wood

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RE:

What is the best way to make balsa wood flexible for construction of a bridge?

I have read methods that say to boil it in water and other methods that say to simply steam the wood. Tell me what you think is best and tell me how much time it takes etc.

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You are correct. The diagonal braces usually connect the intersection of one horizontal and one vertical member to another intersection. The diagonal braces help prevent sidesway in a frame. By using diagonals to create a truss the members of the truss become more efficient because they typically have only axial forces (like pushing and pulling on the ends of a member) and no bending forces (depending on how the loads are applied). Definitely use diagonals on your balsa wood bridge (they don't have to be an "X" they can be "/" or "\").

Easiest way is simply to soak it in warm water until it is saturated and carefully and slowly bend it into the shape that you require. Secure it and let it dry.