If you're asking about the actual format, this is what I submitted and was accepted:
Structure it like an actual formal letter, with a from and to address left justified.
Then, just like a formal letter, start by addressing the committee : Dear Graduate coordinator,
Format the first para as a brief intro stating that you're applying to the program, a little bit about yourself .
In the following paras, interweave information about yourself and your life experiences and why you got interested in OT. Address specifics that the school asks for. My school asked for : "Why OT" and 'Why this university' Be sure to address all they ask for. That will show that you did not generate the same letter for all univs.
I hope this helps and good luck with your application