This is a great question.
Start with the question you have been asked to answer and determine the number of words or pages you have to provide.
Then start with the message. Don't put pressure on yourself to come up with the words.
First come up with ideas.
I like essays that have an introduction, 3 compact interesting paragraphs, and a good conclusion that ties everything together. I usually write the introduction last and I try to come up with a strong introduction that makes the reader curious about what I have to say. Think of the introduction as a newspaper or online article headline. Consider asking a question or making a statement. Try a couple of different approaches and read them out loud or to a friend. One will stand out as the best.
Always write about what you know because you are the expert on yourself.
Consider the program or programs you are applying for and make sure the essay answers the question. Think about how other applicants might answer or multiple ways to write the essay. Choose the one that you are most comfortable with as this one represents you as an applicant.
Read the essay out loud. Have someone with excellent spelling skills read it as well. Start your essay early enough that you have a couple of days to reread it and see if the message fits with what you are trying to say.
Have faith in your ability to craft a message that shows that you are the right applicant for the program.