Craftsmanship during the build process can be just as important as an effective plan during the design process. A well-designed bridge with poor craftsmanship will fail prematurely.
Choose your wood wisely. If you have a scale available, find each piece’s mass and write it on the wood. One idea is to use heavy wood for the main supports and light wood for the cross members.
Now find some cutting tools. Exacto knifes work well. So do miniature saws. When cutting make sure to keep the blade perpendicular to the wood so you get nice clean right angle cuts.
Lay your cut pieces on your design and glue them together. Be careful not to glue your pieces to your designs. Use an alternate piece of paper for all the glue work to keep your designs clean. Or you could make multiple copies of your designs and glue directly on them. Some bridge builders place a piece of wax paper between the wood and their plans to prevent themselves from gluing their wood to their designs.
Add additional wood to your joints to strengthen them. These are called gussets.
To save precious grams of mass, you can sand portions of your bridge. Sandpaper works well, but is slow to use by hand. A Dremel rotary tool or similar device will greatly reduce the time necessary to sand a bridge.