When children are writing an unknown word, one of the ways of solving it is to stretch it out to hear the parts. In Reading Recovery we use sound (Elkonin) boxes to say the words slowly in order to hear each sound and then choose a letter or letters to represent each sound.
I have linked to a You Tube video about using Sound Boxes on the Useful Links page. This is an 8 minute clip that was originally intended for teachers and so it has a lot of ‘teacher talk’. (Click on the picture if you wish to view it.)
Here is another You Tube clip that is shorter and may be easier to understand. (Click on the picture.) Hearing each sound, and hearing it in order, can be tricky for some children. For others, choosing a letter or letters, e.g. sh, to represent the sounds can be difficult.