From DIYinfo.org
Normal interior doors are hung in the same way as exterior doors (see How To Hang A Front Or Back Door), with the following differences.
As interior doors are not likely to swell because of moisture, a 2mm gap between door and frame will be enough.
When cutting the door to fit, make sure that it will open over a carpet without rubbing. It should just brush the top of the carpet.
Alternatively, fit rising-butt hinges (see below)
If you need to trim more than 10mm off each edge of a standard-size door, buy an interior quality panel door. The internal support-frame of a flush door is too narrow for it to be substantially cut back in width.
Alternatively, buy a slightly under size flush door and add timber lipping all round to make up the height and width - no more than 10mm on each edge.
Two hinges will be enough for hanging an interior door. Use 75mm pressed-steel cranked butt hinges
Fitting Rising Butt Hinges
Rising butts are available for right-hand and left-hand hanging, so make sure you buy the right type. Stand so the door will open towards you; if it is to be hinged on the left buy left-hand hinges, if it is hinged on the right buy right-hand hinges.
The part of the hinge with the pin is fitted (pin upwards) to the door frame, and the open spiral section is screwed to the door.
Fit them in the same way as ordinary butt hinges. It will also be necessary to plane a small slope on the inner top corner of the door on the hinge side. This prevents the door from catching on the top of the door frame as it opens.