When I taught P1/2/3 I used a big book as a stimulus for writing. It is a technique called shared reading and writing. The children and I would read the book together and they would complete a writing activity based on the book. The focus for P1 might be writing a sentence with a capital letter, finger spaces and full stop. A good book to look at is "Cat on the Mat." The text is simple and repititious for the children to follow. You then modelled to them on strips of card how to write a sentence with the success criteria in mind. Get them to help you to put in the full stop etc. They then wrote a sentence and drew a picture. I found the children responded well to those sessions and it also ensured all the children were being included as I would differentiate the activity.