As far as I understand you have one Optomcas essay and then individual essays for each school (as to whether other schools have essays to the essays you sent to your other schools is a question i am currently asking

)
but yes so I have a general personal statement on Optomcas answering the prompt as to why I want to be an optometrist, my unique experiences/abilities that'll help me succeed in this field etc
for the school specific essays I first started off writing the essay for my top choice...that was easy because I wrote about how I first heard of the school (for me it was a unique situation) and of the optometrists I have shadowed who have gone/taught at that institution and what I learned while shadowing them, a very small personal anecdote compared to my optomcas essay, and I also wrote about why that school is my top choice, you know like unique reasons and why I would be a good fit for that school and stuff.
Just begin by writing from the heart, no matter how "wrong" your reasons or grammar may be because it'll also be a way you being able to personally ask yourself why do I really want to go to optometry school and then you can tailor it into the beautiful, professional personal statement that you will be ready to submit.
Good Luck