Test the child frequently using flash cards, after going through the alphabet. Show him a flash card with a letter on it and ask him to tell you what sound the letter makes. Make a small notch on the card every time the child misses the sound; so you can remember which letter sounds the child needs to work on.
Play a game with your child that will help her learn to associate the sound with the letter. Pick a letter at random and make the sound, and ask the child to write that letter down on a piece of paper. Draw pictures on a sheet beforehand -- with the letter next to it -- and tell her to circle the picture or letter that corresponds to the sound she heard.
Teach your child catchy songs and poems -- particularly nursery rhymes -- that repeat words often. This will allow you to zero-in on a particular letter sound. For example, you could teach your child about the letter "B" by reading the nursery rhyme "Baa Baa Black Sheep."
Be patient and do these exercises every day with your child. Really driving home letter sounds with your child requires lots of repetition, as with any learning activity. Mix up the activities and poems -- so the child doesn't plateau or become bored.