First of all, you're a kind soul to take on this task....
What's her age? What's her physical condition like (in shape, a bit flabby?)
Sharon has given some wonderful advice that I concur with.
Adequate exercise is essential.... you want her to be in shape for the labor.
As your girl approaches the whelping date, you may or may not notice the pups dropping and her shape changing (depends on how many pups are in there). Her temperature will drop from her normal (around 101F) to 99F (or below)and hold... this is a sign that the pups are soon on their way. It's a good idea to take her temperature rectally now to get a "normal" reading on her.
Check her gums, make sure they're pink and healthy. If they're white in appearance, but you can press on them lightly to bring back color, then she's ok.. if the color does not reappear, she's possibly anemic. Get her checked by a vet.
Make sure she's getting several small meals per day, as the pups will take up space in her abdomen.
Get a whelping box built, (preferably one with "pig rails" so the pups won't get squashed by mama) and place it in an out of the way part of your home. Like Sharon said, draft free, and warm. The pups will not be able to control their own body temperature for a few weeks after birth, so warmth is vital for their survival.
If you find it difficult to buy supplements, I have a recipe that's made with canned goat milk that is very good for the pups.
Here's a good link that helped me a lot as far as being prepared for the day of reckoning... ;)
http://www.woodhavenlabs.com/whelping.html
Hope some of this information assists you.
Let us know how it's going.
Good luck!