The easy way to teach your daughter how to conduct a breast exam is to take her to a doctor or nurse and have the health professional explains how it's done. Alternatively, you can hand your daughter a pamphlet from the American Cancer Society that illustrates the procedure. If you're one of the do-it-yourself breed, you could demonstrate how it's done while wearing your clothes or prepare a bowl of Jell-O, using less water than what the instructions call for to make the Jell-O firmer. While the Jell-O sets, you can insert some grains of rice, a pea, a sunflower seed, and a chickpea in it in different places. After the Jell-O sets, carefully spill the bowl into a freezer bag and close it tightly. Let your daughter try to find the small objects inside by feeling for them (the rice is pretty hard to locate).
If you want, you can sit your daughter down in front of the TV and show a film that demonstrates how to check one's breasts (in the shower, lying down, and in front of a mirror). Although teenagers and young women in their twenties are at very low risk for breast cancer, it doesn't hurt to become familiar with the technique at a young age. If your daughter knows how a normal breast feels, she'll be able to detect if there's ever a difference. Reassure her that most lumps are either cysts or benign growths, but that it's worth it to have any lumps checked out by a doctor for her own peace of mind (and yours).
A health professional will show your daughter how to lie down with one arm behind her head and how to use the middle three fingers of the other hand to check for lumps, then to change hands and do the other breast. Check out the site of the American Cancer Society for more details.