It depends on the phone and what is damaged. If the phone's screen works but is cracked, then its most likely just the digitizer which in many cases is glued to the glass. If the screen is not working at all, then you may need to also replace the LCD screen. Some digitizers are also glued to the LCD necessitating replacing both.
The repair difficulty factor depends on the specific phone. Some parts and repair services provide detailed videos on screen replacements. I'd refer to these over those videos uploaded by users who may be doing this for the first time.
Click the above for a detailed screen replacement video(44 min) for the HTC EVO 3D / HTC V 4G.