Some Chihuahuas have no problem at all with their ears coming up. At about five weeks they just magically pop up and no issue at all. Others take a little work.
I had a friend suggest to me at 5 weeks to cut a piece of the stiffer 3m paper tape (called "Transpore") to the shape of each puppy's ears and just use the sticky side of the tape to put into the ear. You would not make the shape where it went down any lower into the ear than where it is open in the front - stay above that. These will fall off and are barely noticeable to the puppy and when they do, I will check to see if they are completely up. If they are not, I will do it again (same thing). The great majority of puppies have their ears up after this and never go back down or have a problem.
However, some puppies do have bigger heavier ears and they need extra reinforcement. The good news is, these are the puppies who usually have HUGE beautiful ears when done. The bad news is, you'll need to stay on top of this if you want the ears to be up.
The supplies you will need for this later more intense type of ear bracing are skin bond (cement for skin - human product, available on Amazon), the stiff brown paper bags that you used to always get at the grocery store before plastic took over, and Original small/med Breathe Right Strips (clear) for snoring. I am going to put a picture of each to the right so you can see what they look like.
So, the first thing you are going to do is take the paper bag and you are going to cut it in the shape of the ears, where it will fit right down to where the ear opens at the bottom but no further than that. Might just need to hold up to ear to measure or you can actually measure and you will be cutting a triangle shape.
Then you will take the breathe right strip out and you will cut it in half - you will put one half on one paper bag "ear" and one half on the other. You just use the sticky from the breathe right strip to have it stick. You will then put the skin bond on the side of the paper bag ear that has the breathe right strip on it and you will "glue" the paper bag ear into the puppy's ear. The skin bond does not hurt the puppy. Over time (or with a littermate's help!) the paper bag ear will just fall out. You can then look at the ear to make sure it is standing - if not, repeat til it is. At the first sign of ANY weakness, this has to go back in immediately.
I have had this work even on an older puppy that should have had some assistance with ears and didn't. Since I have started this technique, I have never had ears that failed to stand (knock on wood) if this is followed and kept up with.
This guy right here has the 3m Transpore tape in his ears.
The 3m "Transpore" tape to use for 5 week+ puppies
The skin bonding cement. This is what they have on Amazon right now. The brand I usually use is "Osto-Bond".
This is the type of Breathe Right. Be sure to get the small/medium, the clear, the "original." It's what works.
Prepared "paper bag" ears...you see how I have cut to shape of puppy's ear - doesn't have to be exact or fancy. I have then taken the Breathe Right strip, cut in half, and used the sticky part of the breathe right strip to stick on the paper bag ears. I will then paint THIS side with the skin bond and I will stick into puppy's ear. This way, the edges of the breathe right are not on the outside where puppy can easily get them off. The stiffness of the paper bag will also help.
Here is how it looks in the ears to do the paper bag method with the breathe right strips.
This product, TacAway, will remove the skin bond when you are done and the ears are fully up.