Chihuahua potty training is one of the most important things your puppies must learn. If you do it right in the beginning it may only take a few weeks, but if you mess this up in the beginning it will take months to get it right. What to do to prevent a mess in the house to get started.

The first thing to do is watch your dog, and you'll notice a time in which they decide to go to the toilet. This normally is after snacking or eating, so be aware of this and plan your Chihuahua potty training schedule around this. Make sure that meal times are at the same time each day and snack time should be a meal time that is spread out several times a day.
For small dogs like Chihuahua pups, you should allow freedom to roam the house. But for "Big Dog" type dogs this isn't an option you should allow. At all costs avoid allowing your Chihuahua to roam the house when you are not able to directly supervise it. This is where crate training comes in handy. You can train your Chihuahua to go to the bathroom on a puppy pad or outdoors. Just be sure to prepare the area first with the puppy pad or newspaper so that when your Chihuahua goes to the toilet in the right areas they are only, hopefully, on the puppy pad or newspaper.
For your house, well, you need to be careful as Chihuahuas are pretty stubbornly hold their urine and poop until they reach the urinate stage. So you need to be prepared to deal with accidents inside the house. Chihuahuas are the type of dog who holds their waste the longest. You need to read the paper or puppy pad far in advance of when your Chihuahua is due to go to the toilet. One of the ways you can prepare your house for the arrival of your new Chihuahua is to limit the area accessible to your puppy. This is especially true if you have small children who may decide to play with the puppy, although that is their prerogative.
The best way to control the flow of air in and out of the house is to install a pet door. Then the air will only carry the odour to the door and not all the way to the end of the paper or other surface. Then when your Chihuahua potties at the door, it will on the paper or pads. You can then just lift the door and let your Chihuahua be gone.
If you want to give your Chihuahua a little extra privacy then you can use a crate, but that is about the size of a small postage stamp, and be sure to roll out the sides so that the door won't fold in on itself. Most pet doors are equipped with a small flap type door which can be opened from the outside, similar to a back door or window.
An alternative to purchasing a pet door is to allow your Chihuahua to become a full grown dog and have an expandable doggie door on the main door of the house. If you do this, be sure to remember that your puppy will grow very fast into a full grown dog and the doggie door could easily be destroyed. I would suggest buying a large piece of cardboard and cutting it into the exact size of your door after you cut it to size. glue it to the inside of the door with the string you cut to attach it.
Right now your Chihuahua can use the bathroom on the carpet, but it will take a little longer to get through than a baby gate, so that's something to think about.