Like pearl said, research about the school you gonna send
For example, you don't want to send Marquette as they are done with their interviews right now.
Send to schools that are not done with interviewing yet.
However, even if chances are slim, you should try regardless and I think you should send a letter of interest to all the schools that are still interviewing, because why not?
What I wrote in my letter of interest differs from school to school, but not that significantly.
I just made sure to point the strongest factors of each school I contacted.
I wrote the letters to their office of admissions or sometimes for the admission director.
Dear Office of Admissions,
The first paragraph is greetings and an intro my name and when did I apply and how I have contacted other students of each school and learned about their program
The second paragraph about why their school means so much for me.
The third paragraph I update them about what I have been doing to improve my application and education
The 4th paragraph I write a personal reason why I want to live in that city
Finally, I thank them for their time and help
Before you send them, make sure to proofread what you have written
and proofread it again
and again ( because you don't want any mistakes)
You can show it to your friend to see if he finds any mistake there.
You can show me your LOI when you are done if you want.
and done to the next school. Do the same.
Best of luck and meanwhile keep on improving your app for next year just in case you did not get in.