Once you have the pontoons ready, you really just have to lash the boats together with rope. They’re boats, after all - they can be constructed first, and then rowed into position; rather than being rowed into position and then the bridge bits hammered on top.

In this scenario, the crossing was ultimately uncontested - when the bridge was constructed and the first troops starting moving across, the enemy force scattered.

That being said, there’s a reason that contested crossings aren’t the norm even when you have a permanent bridge in place - very dicey to go through with!