Just open up your DS (there is a disassembly guide on GBAtemp), unplug your touch screen (and probably the lower LCD too) open the black tab before trying to pull the cable out.
Then you will need to CAREFULLY prise apart the touch screen and LCD screen (unless you are going to replace both)
Then stick the new touch screen in place, and plug both the ribbon cables back in, making sure to close the black tabs.
- You do know the touch screen and lower LCD are two seperate parts? I know it isn't that obvious.