By reading with them. On your lap with them pointing to pictures, then words then letters. Interaction with parents is desperately important. There are books now for all age ranges, and some of the starter ones can even be taken in the bath. Make it fun, give it lots of attention. Kids are attention junkies, so if they work out that reading=attention it wont take them long.
I found Letterland series a great starter for letters and sounds...I spent summer holiday with my 5 year old and taught her the alphabet using flash cards and memory games and then moved onto the Ladybird reading course 13 levels ....take your time .....read out loud ....start with word families...top,mop,hop,etc....probably the most rewarding bit of work I have ever done...watching my daughter now read on her own fills me with gladness...