My son is 20 months old as well. I start with books that he likes with small sentences pointing to each word then I make flashcards and word wall charts of the words in the book. But now I am a stay at home mom so I have the time to do these things sometimes. But just relaxing with your child and finding the books he/she likes best, like my son loves cars is a good way to start, curling up in bed or on a rug. So I try to find books about cars. Also here is a great website for books to print that is a teacher created resources that comes with the word walls and everything for you already done.
http://www.carlscorner.us.com/Readers.htmI started off with books about his toys. I took pictures of his toys and put them into a power point slide show then printed them placing the book on shower curtain rings. He knew these items were his and he would go and find the object that i presented. I also made posters of the toy and their names under the picture and hung it up in his room where he could see it while he plays. He now points to the poster or word walls and says the words or the sounds of the pictures. Like for car he says go or v-room.
When he plays while i am cleaning house I hear him mumbling words and he pretends to read a book sitting on his favorite pillow under the window seal pointing to the words as he pretends.
Here is another website as well for online reading tha my son loves
www.starfall.comalso never try to teach a child when they are sleepy or ill it just frustrates them and then you to. My son when he gets tired wants to touch and tear up everything even his books some times.
I also found out that he likes books that are small, that fits in his hands well, and his books have to be very sturdy because he loves them so much, he wants me to read with him all the time.
For math we have books that you count with cherrios, he loves it.