Residential metal doors are not that different from typical flat slab exterior doors often seen on houses. They are not solid metal; the exterior metal cladding typically is only 1/16-inch thick. The interior of the door is made up of solid wood framing with fillers and some void space. Generally, these doors come framed in their own jamb and do not need to be adjusted, but retrofitting may require you shave off an inch or two to make the door fit.
Remove the door from its hinges. Using a hammer, drive a screwdriver up through the hinge to knock the pin out the top, starting with the bottom hinge and working up. Work the door free of the hinges by opening the door and lifting up on the knob while keeping a hand on the door's middle to balance it.
Place the door flat on a pair of saw horses or other sturdy work surface.
Measure the door frame at each corner, and through the middle from the bottom edge of the jamb header to the floor--or to the top of the threshold for exterior doors. The smallest of these three measurements is the reference you need for cutting your door to size.
Measure and mark the door from the top down along each edge, 1/4-inch shorter than the shortest measurement of your jamb. Wrap a piece of masking tape around the door with its center in line with the two marks. Re-measure and mark the door on top of the tape. Use a chalk line or drywall square to mark the door between the two side marks.
Use a circular saw with a bi-metal blade to cut along the bottom edge of the line, removing the marked section from the bottom of the door. Sand the bottom edge of the door smooth with a belt sander and remove the tape.
Return the door to its hinges and tap the pins back into place, starting at the top hinge and working down. Test the door and re-cut if needed.