Learning about rhyming sounds will help children become phonemically aware. Start with a some simple rhymes and see if children can supply the missing last word. For example say, "Hickory dickory dock, the mouse ran up the . . . " and see if children shout, "clock" or another word that rhymes with dock, if they are unfamiliar with the nursery rhyme. Have them help you make some simple rhymes or read a rhyming picture book leaving out the last rhyming word on each page to see if they can guess it.
A key element of phonemic awareness is understanding that words are made up of syllables. Have children use toy drums to beat out or their hands to clap out the syllables. Start by looking around the room at simple objects and have them beat the number of syllables. For example point to the window, and show children how to beat or clap on the "win" syllable and the "dow" syllable. Clip some pictures of animals or other familiar objects from magazines and use those to have children beat the syllables as you show and say each one.
Each word has a beginning, a middle and an end sound. You can help children understand this concept by using some sound substitution riddles. Use simple words and ask children what happens when you change a letter. For example, ask what happens when the "t" sound at the end of "cat" is changed to a "p" sound, cat becomes cap. Try asking children for rhyming words that start with a different letter. For example, what rhymes with "book" but starts with a "c" sound.
To get children used to putting the parts of a word together, play a simple game where you say a word slowly and see if the children can say it fast. For example, say the word cat very slowly by sounding out each letter. Children should recognize the word and say it fast or at a normal pace. Start with one syllable words then progress to trickier two-syllable words.