My favorite way to teach letter sounds is with toys and some block or foam letters (we use the Melissa & Doug Jumbo ABC Chunky Puzzle daily). The game is simple.
1. Lay out three to four letters out on the work space.
2. Using the letters in the photo as an example, say "fff" (the letter sound) and hand your child the flower and say "Flower. Flower starts with 'fff'." It helps to emphasize the initial sound a bit.
3. Then hand your child the letter and say "ffff. This is how we show the 'fff' sound."
4. Repeat for the other letters.
Next time you sit together, lay out the same three letters. Hand your child one of the toys (the flower in this case) and say "Flower. Can you find 'fff'?" If he can't, just show him. Have him hold both the object and the letter at the same time so he can make the connection that there is a relationship between the two objects and so that he can feel the general shape of the letter (Foam Bath Letters are better in this case).
You can also do the reverse and hand him a letter and ask him to find the object that starts with that sound. " I spy something that starts with 'fff'."
Every child is different, but I started this game with both my children at around 14 months. At 15 months, J-jo just likes to hold the letter and object together and try to imitate the sound. By 20 months, Bear could always match them.