All you have to do is cut out two curved sections from some foam board. Next cut out a long rectangle. It doesn't hurt if it is too long. You can trim the excess when you are done.
Once you have your three pieces cut slits in the rectangle so it will bend. This sounds harder than it is. You have to press really hard to cut all the way through so don't worry about it. I used a ruler to make sure my lines where straight. I didn't worry about spacing so much though.
Once you have your piece scored bend it to the shape of your curved side pieces and glue it in place and secure it with pens. Seal the exposed edges of the bridge with glue. I didn't worry about sealing between the slats I cut to allow it to bend. Toss it aside to dry. Work on something else.
Once it has dried just spray it with spray paint and finish it up like the rest of your terrain.
It is as easy as that.