My advice would be to start with potty pads (I also use washable ones) and then introduce going outside. Until Colt has his last Parvo shot it would probably be safer to carry potty pads just like diapers and have him pee on those even outside. I felt silly doing it, but I have a healthy 10 month old dog instead of a sad story.
She has no trouble doing her business outside now and there was no real work involved in teaching her.
My other dog was never trained to use potty pads and it's been.....interesting. She has bigger issues that take precedence but it looks like it's going to be a long road if/when I get serious about potty pads or just accepting some accidents as an inevitable part of life if I don't.
Since you are already a Chihuahua mom, I assume you know how vulnerable our tiny babies are to hypoglycemia. I felt better knowing I had a tube of Nutrical handy at all times when Laurel was that tiny even though she never actually needed it.