If you are considering doing it simply for fun, don't, there are plenty of babies out there already, many end up in shelters or rescue centers, still others end up on the side of the road where they are easy pickings for predators.
If, on the other hand, you got your piggie from the pet store, and never intended to have babies, but find your little pet pregnant ... don't panic. Cavies have been having babies for a long time, even very young ones can do a good job of being pregnant and have healthy babies that are healthy themselves. So all you have to do is ensure that you provide the best possible environment for the expectant mother.
Since the mother is now eating for more than one, you need to ensure that her diet is substantial both in quantity and quality. You should be feeding her a high quality pellet, without all the fancy "junk" in it. The pellet is a balanced nutritious food for the cavy. In addition, she will need additional Vitamin C and Calcium, this can be dealt with by adding such foods as spinach, kale and parsley to the diet, or increasing their availability during the pregnancy. These are high in both required nutrients. Her water bottle should be kept as clean as possible, with fresh water given daily. In addition, you might want to add a small bowl that contains karo/molasses water. This water provides some added sugar to the diet (not generally recommended, but helpful in this situation). The water is made up by adding about a teaspoon of karo/molasses to 8 ounces of water. Keep the unused portion refrigerated. This sweet water seems to help prevent pregnancy toxemia.
Gestation is 65-70 days. At about 3 weeks before birth you will be able to feel the babies moving. A few days before the birth the pelvic girdle will separate to allow the babies' passage - a side effect of this is to make the piggie look even flatter than she did before! You can check the pelvic separation by running your hand down the piggies tummy, using the longest finger to go down her centerline. When you get to the vaginal opening, just forward of that you should be able to feel a boney bump. This will separate into two bumps shortly before the birth. When it gets to be about a finger's width apart the birth should occur within a few days.
Labor is somewhat hard to recognize. She will bend over like she is licking her lower stomach and may appear to have the hiccoughs. She will deliver the babies, each in it's own sac, she must open the sac over the babies mouth/nose so that the baby can breathe; if she doesn't do this, you can open it for her. She may start to clean each baby as it arrives, booting it about with her nose to get it started breathing. If the babies come out too close together she may get a little confused and fail to open the sac over one's mouth. If you are there, do this for her, but do little else. Once all the babies are born she will clean them up and probably eat most of the afterbirth. This not only stimulates her to lactate (babies have enough reserves to survive for a day without drinking mother's milk) but also removes the evidence so no predators can find the birthing site.
The babies will be wet for a couple of hours. Once they are completely dry you can handle them. The mother will not reject a baby because of human contact, especially if she has become use to human contact herself. So you can handle the babies the first day of their lives. Enjoy this time, they will not stay tiny for very long. The babies will suckle for three to four weeks, at which time the mother will wean them away from her milk. For a large litter, you could add a milk sop, this is simply bread soaked in cow's milk and provided at room temperature. Do not leave it in more than an hour or so, the milk will sour and may harm the babies. Don't be surprised to see mom eating the milk sop, it provides her with needed energy and nutrients too. They will start eating the same foods as mom very early, some will attempt to eat pellets or veggies the first day. Remember, they are born "precocious" meaning fully furred, eyes open and all teeth and nails functional. They look like miniature adults!
The boars should be removed as soon as they show signs of sexual maturity, this is indicated by them starting to "rumblestrut" or do the mating dance. While their sisters may be safe from becoming pregnant, mom isn't! So remove the boars early.
Once the babies have been weaned you should immediately separate the boys from the girls. While it is unlikely that they could impregnate their sisters, it is a good idea to get them out as soon as possible. Many of the breeders I know prefer to keep the little ones around until they are six weeks of age before sending them to new homes. But do not leave the boys with the girls for that long. Boars should be removed from both mother and sisters at about 3 to 3 1/2 weeks.
If you intend to breed the mother again, she should be given a minimum of a month to recover her strength. This means a month after she is finished weaning. Many of the breeders I know do not breed a sow more than twice a year and retire her at about age three. She then gets to be a special matron of the herd! Other things to consider are how her last pregnancy went. If she had a hard time; late births, large single baby, serious medical problems during or immediately following the pregnancy, then it would be better to retire her early.