The type of touch screen found on an iPod touch is known as a "capacitive" touchscreen. What this means is that the screen creates a tiny electrical impulse able to be picked up by the human body. This makes it perfectly responsive when a human finger is used to navigate the screen but causes it to ignore a normal plastic stylus like the one for the Nintendo DS. In order for a DS stylus to work on an iPod touch, it will have to be modified to affect capacitive screens.
Cut a length of sponge roughly half the length of the DS stylus and about four times as thick with your scissors.
Penetrate the length of sponge with the stylus, almost like sliding on a pen cap. You may need to cut a small hole at one end of the sponge if the stylus cannot penetrate the sponge on its own. Insert the stylus as far as you can, since you need to end of the sponge to be as firm as possible.
Leaving about 6 mm (1/4 inch) of sponge protruding from the end of the tape, secure the rest of the sponge to the stylus with a generous wrapping of adhesive tape. Wrap the tape around several times to create a secure hold.
Wet the protruding edge of the sponge with plain water. When wet, the sponge is capable of conducting electricity and will respond nicely to a capacitive touchscreen. Your DS stylus has now been modified to work on your iPod Touch.
You will have to keep the sponge moist for it to work on your iPod touch. Keep a small glass of water handy to avoid repeated trips to the faucet during prolonged use. If the sponge begins to mildew or stink from repeated use of water, simply unwrap your stylus and use a new piece of sponge.