I like to make stuff from cardboard. from cutting down a big box to a smaller one for shipping, to making props, storage solutions etc.
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 used a scrap of MDF 1x6 trim board. Mostly because it's what I had and it's easy to work. also it's stable and wont cup or twist.any wood would work if it is flat. solid wood is stronger and hardwood probably would last longer.
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

Simple step, cut the curve. I used a jigsaw, but you could use a coping saw, scroll saw, bandsaw, or whatever you have. Probably not a chainsaw.

Step 3: Profile the Edge

You now need to make the curved edge a sharp V. this is what creases the cardboard for a neat bend.. I did this by hand with a block plane. You could use a router with a chamfer bit, or just carve it with a utility knife. You could also have tilted the table on your scrollsaw or bandsaw when you cut the curve. The angle is not critical as long as the point is fairly sharp and straight.
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.

Now just get your piece of corrugated, and mark a line where you want to bend it. Start with the tool rocked up on one end of the line and roll it down the line. that's it!
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.