Different employers will have different instructions or expectations with regards to supporting statements, but generally they'll be looking to see if you're applying for the right reasons and that you meet the essential criteria.
If you haven't already done so, read the general advice on writing job applications first.
To help give you an idea, have a look at our Sample Supporting Statement, which has been written to to correlate to a sample Person Specification and a sample Job Description.
In particular, pay attention to the way the Sample Supporting Statement:
The sample should be used as a guide only, and when writing your own, you can use a structure, layout, style and format that works for you.