I'm assuming that the NDS in question is an original NDS not a DS Lite
Look at this site http://site.powergamingparts.com/RepairNDS.html
which tells you how to take one to bits, you only need to concern yourself with removing the motherboard bit the bottom screen will then lift out, the touch screen is stuck to the bottom screen, carefully prise the screen off then put your new touch screen in the case on it's own make sure it's the right way round if you look closely while doing it there are small guides to place it correctly then you just need to put the LCD screen on top of it this I've found is the easiest way of making sure it's put on accurately. Then follow the instructions given on the site in reverse!
Hope this helps
Jon Nash
Any questions you can contact me at jonashuk@aol.com
Jon Nash, January 2008