Cardboard can be a little bit tricky when it comes to getting crisp and clean folds right where you want them. Here are some tips on how to do it.

Take a look at the side of the cardboard sheet. See those flutes? To get a crisp bend you have to cut right in the valley of a flute. The black arrow shows where to cut.

I have drawn a line here on the cardboard to show you the cut to make right inside the length of that flute valley.

Go ahead and cut along that line, only cutting through the top layer of paper.

And then run a pencil through that cut line to open it up.

Now you can easily fold it and achieve a nice crisp corner.

You can see the nice and crisp edges on this structure I made using this process.