here are the steps you need to follow

Disassemble the cardboard box and flatten it like this:

line a bordor of a side of a bridge with a pencil on the cardboard (do it twice)like this:

cut the cardboard along the bordor, then you will get a side of a bridge like this:

cut some rectangular pieces of cardboard for the stairs and the fence

glue the pieces of cardboard between the two sides of the bridge and glue the fence on the top of the bridge

now you have a bridge