The hard part with corrugated cardboard is in bending it nicely. someone else has shown how to cut the inside ply and corrugations to make a neat bend, but I have a problem with that method. #1 it's weak, and #2 Have you ever seen a commercially made box made that way?
I used to try holding a yardstick down on it and bending up that way, but it was very unpredictable and often I got poor results. At some point I took a straight board and cut a bevel on one edge to crease the bend. it worked sorta. since the pressure was spread over a large area, I had to really lean on it hard. the sharp square end made it hard too extend the crease if a long fold was needed.
Recently, I had an epiphany! If a made the creaser curved like a pizza knife, it would apply the pressure to a small area at a time and it would require a lot less force to do the job. Also, if I needed a long fold, the curve would let me ease out of and back into the work for a smoother transition.
Step 1: Lay It Out
I cut a piece about 18" long and about 4 1/2" wide.
To mark the curve, I put a small nail about 2" down from one edge, one on each end of my piece.
I then used a thin strip of wood about 1/4" thick and 3/4"wide as a spline and layed it against the two nails then bowed it to the curve I wanted and marked it with a pencil.
Step 2: Cut It Out
Step 3: Profile the Edge
To start, I used a pencil and finger guide to mark a line about1/8" from the edge both on the edge and the face. then I planed to the line. Now I flipped the board over and planed a bigger taper to meet that one and make the point.
On the first model, I made the V centered on the board, and it worked, but I had to stand on my head to see the line. Thus on this model I moved the point over close to one side for visiblility sake.
For durability, you might want to soak up the mdf with some shellac or wood hardener, but I didn't. it's simple enough to make another if I wear this one out.
Step 4: Try It Out.
If you need a longer fold than the tool, just move it over and start again.
Now that you have it creased, just grab your cardboard and fold. NIce straight clean bend every time.