First, be sure you like your hinges. We changed out the metal of our fixtures, towel and shower curtain rods, even our swithplates, so it made sense to swap out the 1970's brass door hinges for satin nickel. This was a relatively easy swap, though it helps to take an old hinge (or at least a picture of it) with you to the store. There are several shapes/sizes of interior door hinge to choose from.
Then, measure your door. I did this by measuring the door frame in two places: first, the distance from the bottom of the lower hinge to the floor, and second, from the bottom of the door latch hole (totally not the official name...) to the floor. In theory, when you transfer those measurements to the door, you can draw a straight line across and trim the door at that line for a custom fit.
Third, cut your door. We actually tried to just sand the door down. We're working with ancient hollow-core doors and I didn't want to damage the hard wood spacer in the bottom of the door. This didn't work. Eventually, the jig saw had to come out. When you cut a door, particularly this kind, you should definitely tape the door to keep it from splintering. I did the first few times (see below for details on how many times...), but got lazy (frustrated? impatient?) towards the end and splintering occurred. Luckily, what I added didn't come close to the splintering already going on with this door (remember, it's old and we've no idea what the PO did to it), and we're planning to eventually replace it anyhow, so I didn't really mind too much.
I'm linking up with Beth's How-To party over at The Stories of A to Z today. Head over there for some fabulous tutorials!