Why do it?

It's often useful to introduce students to the requirements of the exam, so that they understand what it takes to write (and it is usually write) a successful piece of examined work.

Past Answers

You could start by showing students a question, and giving them an example script of a successful student.  Often the best scripts to use are those who have not written a great deal, but have been awarded high marks, so that it's easier to see exactly what the candidate did that was so successful.

It could be that you give the students an un-marked script, and a markscheme (perhaps simplified if necessary) and ask them to have a go at marking to a level.

You could give them an script or an unmarked script and ask them "what's good" (or bad) about this answer, before moving on to marking with a script.

Their Own Work

You could ask students to complete some work in exam conditions, then give them a mark scheme and ask them to mark each others work (expect groans and even aggression the first time you suggest this).  At  this point you could introduce an exemplar script that you've previously marked.

Overcoming Student Reluctance to Peer Mark

1. In an ideal world you will have been referring to success criteria, so students will be happy with this idea. (!)

2. Start with marking their own short pieces of work, using simplified markschemes, and teacher marked exemplars.

3. Suggest improvements for themselves, using the markscheme.

4. Move on to marking each others, and suggesting improvements.