Most shipping container customization plans will involve adding windows, doorways, vents, or even opening up a sidewall. Cutting Cor-Ten steel, although not as easy as cutting wood, can be done with several different tools that you may have available in the typical handyman's toolbox.
First and most important is to protect yourself while working with cutting tools. No matter what tool you select, the cut steel will have very sharp edges; wear a pair of heavy leather gloves like those designed for welding. The cutting operation involves sparks and metal shavings; wear eye protection! The tools are loud, and the steel shipping container will increase the noise; wear good hearing protection! Also, wear a heavy long sleeve shirt and long pants.
That may not always be pleasant; however, it sure beats hot metal shavings landing on you. Regarding landing on you, shipping containers are constructed from 12 gauge steel, approximately 1/8″ of an inch thick. A big piece of this stuff, like from a door opening, will be heavy, be careful when finishing your cut to ensure a heavy piece of metal with razor-sharp jagged edges will not fall toward you and injure you.
To begin, use a sharpie-type marker to draw the edge of your opening on the container steel. If cutting an opening on a shipping container isn't or will not become a frequent practice, you can use a saws-all or reciprocate saw with a heavy steel cutting blade. Save yourself some frustration here, don't buy cheap blades; they're cheap for a reason, this is heavy-duty work, and you want several heavy-duty blades available. Drill a hole inside the line of your marked opening big enough for the saw blade, put your blade in the hole, and begin cutting along your line.
You can also use a skill saw with a good meal-cutting blade. To start your cut, first set the blade on top of your mark to get lined up. Lift the rear of the skilled saw enough so that the blade leaves the container surface and starts the saw blade; slowly ease the saw blade down to your mark until the blade cuts through the steel, then follow your line. You'll have to repeat this process when you come to the corners, as a skill saw does not turn. Don't panic in the event you wander off of the line a little bit with either of these cutting methods, any error can easily be fixed.
Those who cut this type of steel regularly should have a torch available or the ultimate field-cutting instrument, a plasma cutter in the tool arsenal. These will be much quicker and simpler than saws and much less fooling around when turning corners. You can rent these tools at most equipment rental locations but practice on some scrap metal before you begin cutting on your container.
While it's heavy-duty work, cutting openings in shipping containers isn't hard and can be accomplished with any number of tools; just do it – carefully, and the easiest for your method will come to you.