The first step is touring. This is usually available to anyone three years before the application year. The point of touring is to become familiar with the school's campus. It is recommended to tour as many schools as possible, as this step is not stressful. I was able to tour about a dozen and found it far worth it.
After touring, presentations are available. Presentations are a bit more informative and cover the curriculum and amenities of the school.
I highly recommend taking a special class to prepare for the placement tests. The tests are simple, as they are standardized, though it is helpful to know the format and how they are scored. The class mainly involves taking practice tests and learning how to pick the right answer whether you know it or not.
In addition to the placement test class, it always helps to have an essay coach. The coach will advise on what to include in the essays and how to elaborate it. Although the essays are somewhat short, they are time consuming. The challenge is incorporating everything necessary into a compact yet detailed paragraph.
Next, shadow days become available. A shadow day is a final visit in which the visitor gets to see what it is like to be a student at the particular school. I enjoyed shadow days the most out of any other step since nearly all the schools let the visitors participate in lessons and art projects. Be ready to write an essay at any moment during the day.
After submitting the essays and other information with the application, interviews will be scheduled. So far, I've had two and they both were too simple. I realized that I had wasted my time memorizing the schools' websites when I found out they wanted to be very casual. Even though they may be called "interviews", they are merely conversations. The counselors may ask you about your interests, talents, and academic strength.
That is the shortest possible way I can summarize everything. Remember, it is never too early to begin finding the right high school. Ask me in a month about acceptance letters.