It's very possible that writing will just 'click' for them in year one, but there's a lot that you can do over the summer to help:
Lots of reading, not just books but signs, leaflets, anything with writing on!
Let them see you writing very often- write shopping lists, labels, notes for other family members. When you do it, occasionally 'sound out' a word as you write it, to reinforce the idea that words are built up through sounds.
When they play, try to introduce writing e.g. my 4 year old loves doing shows, so I might suggest that we make a poster or a programme. It doesn't have to be an essay- at this stage 'The show is here' is plenty. And again, if they're not keen on writing, you can do it. (You could always do one sign and then leave a pile of paper 'just in case they need any more'!)
Try writing in the sand at the beach, or in paint, or using water on the garden fence, or with a sharp stick in play dough.
Use magnetic letters and make up notes from them on the fridge. Try asking questions so that they can answer- shall we go to the park or the shops?
Try writing postcards or letters to friends.
But don't push it too much- you want to make it fun, notput them off.