Teach Students How to Write a Topic Sentence.
I have found that having an anchor chart with sentence starters for students to refer to is super helpful. I like to create the chart in front of the students. It might look something like this:
Go Slowly and Progress in Small Steps
Use the Discussion Web Strategy
Usually, the hardest part for first graders is thinking of good reasons for their opinions. This is a simple strategy to use. I like to gather students around me by the easel. Start by having a question in the middle of your chart, and two columns on the sides. It should be a question that can have strong opinions on both sides. The chart above has More/Less as headings, but you could also have Yes/No, or other headings as needed. Then lead the class in a discussion of both viewpoints. List student responses on the chart. Now, students are ready to complete an opinion writing piece on the question.