DD's a pretty good talker. She's just started coming out 4 to 5 word sentences at 19 months, which I'm thrilled about. I'm just wondering at what age your DCs were introduced to a bit of the old ABC? She kind of knows what letters are (although at this stage she's better at recognising numbers in their digit form).
And also, how did you start? The ABC song? She has a foam alphabet in the bath but I don't think she relates it to anything yet, they're just colourful shapes. Fun first, obvs.
I started teaching mine letters when they were interested in them. For DS1 that was at 18months ish, he certainly new all his letters by the age of 2. For DS2 that was when he was nearly 5, just before he started school. For DS3 he was around 2ish, he doesn't know them all yet as he gains and loses interest.
I wouldn't start with the ABC song, but with the usual sound that a letter makes. Hard to write down but mmmmm for m, tuh for t etc. I knew mine were interested as e.g. when we were walking down roads for example they would ask what the road names said. By the age of 2 DS1 wouldn't eat a yogurt without knowing what every word on the pot said. He was aware from a very young age that he couldn't read and he wanted to. DS2 on the other hand had no interest in letters so there was no point trying to teach him them.