Read books everyday, constantly. In between physical activities, or whenever there's down time. If you've read a particular book to them a couple times already, start redacting nouns/proper nouns when reading it again–and let them guess what the word is (reinforces critical thinking and word memory).
Introduce books written in other languages so they get a sense of the diversity of human language. Ideally it would be a second language you're familiar with, in order to read it to them–but exposure via recording or video is beneficial too. For example: Japanese Nursery Rhymes: Carp Streamers, Falling Rain, and other favorites, a book of children's rhymes that presents each line in both Japanese, and an English together.
Walk through and translate a Children's book together – うらしまたろう for example.
As early as it's feasible to hold a child's attention for this (possibly around 2.5–3 years of age), introduce them to 'long form' chapter books. Try to get through one chapter per reading session so you can help the child solidify mental landmarks in the story, and recap each time you read a new chapter together. For example: The Lion, the Witch, and the Wardrobe.
Sing the ABCs song every night together twice while brushing our teeth (one time per row).
Beyond walking through words in books together, flash cards are a fun exercise for comprehending how letters from the ABCs come together to form words.
- Pooh's First Words Flash Cards: Learn to associate verbs and nouns with fun characters kiddos like.
- Mr. and Mrs. Potato Head Alphabet Flash Cards: Gain an understanding of 'big' and 'little' letters (upper and lower case).
Pose the question: 'What is an Alphabet?' and guide them towards understanding that they are:
- A group of letters.
- A set of letters (reinforcing the concept of a set)
This can be asked during flash card sessions. Have them repeat the answer, and also show them alphabets from other languages to support the concept – for example: a hiragana chart (Japanese).