1. Write your own . Start with why you really want to do Phd. Attached please find a guide from HBS phd pages, should give some pointers.
2. BE extremely positive and attacking. Write about why PhD Career option excites you.
3. Show it to your friends, ask for critique.
4. Do some research about the school. Please do not assume that a single SOP is fine with all schools - it is not. Your SOP must indicate how THAT school fits your interests and whyyou are interested and excited about that school