We follow the Jolly phonics programme in our school. At the moment, the children are learning one letter sound per week.
It is so important to revise this sound EVERY night, as well as revising all previous letter sounds. You don't have to do this as homework all the time, you could sing the songs when you are out for a walk, play I-spy in the car (but instead of saying the letter name, make the sound of that letter) make a little display in their room of things starting with the letter of the week. There is a jolly phonics app available for download, the first group of letters are free. There are videos on you tube and there is plenty of supplementary material available to buy online and it bookshops.