Usually they need no help. Unless they are long legged adults. Just peek in on them every few minutes. They will be ok.
I'm not trying to be rude, but judging from your question, you have No Experience in breeding Chihuahuas. Chihuahuas are known to have complications during Whelping (giving birth). Some have to have C-Sections, because the puppies are too large for them to deliver themselves, a puppy may be breach (feet first) or they may have a puppy stuck. It is very important that you never leave your Chihuahuas side. The poster above me said that they usually don't need help and to keep checking on her from time to time. That is WRONG! I used to breed Chihuahuas and I have #15 Chihuahuas of my own now. I never lost any of my females, I never lost any of their puppies and since I knew how to assist the mother, none of them had any complications. Before I started breeding, I did a lot of research and knew exactly what I was doing. The first thing you have to do is to take your Chihuahua to the vet, have an x-ray or ultrasound done and find out how many puppies your female is carrying, how big the puppies are and if the vet thinks she will be able to deliver the puppies on her own, or if she will need a C-Section. That is very, very important!!! Do you know what breed your Chihuahua was bred with? If the male is larger than your female, it is almost guaranteed that she will have complications and she will need a C-Section to save her life and her puppies lives. You should have never bred her with No Experience! If she does have complications and you don't know what to do to assist her in getting through the complications or you don't react fast enough by getting her to the vet for a C-Section, then she can die and so can her puppies. There is way too much information to write down here, that you should have researched before you bred your Chihuahua. Make sure that you have your vets phone number stored on your speed dial. Make sure that the vet is standing by, when she goes into labor. When the vet says it's safe, please get your Chi Spayed, so this never happens again.