This is based on our experience.
Most important, you must know what you want to study, i.e. your major - the 3-year course combines curriculum from undergraduate and graduate levels. That means you should demonstrate, through focused experience and effort, that you have been pursuing your interests. Some disciplines are easier to get into than others, e.g. Classics v. medicine. After that, there are many hurdles and requirements. My d was interested in archaeology, so we financed 2 internships in Israel, early Christian. She also read a lot of early Christian history on her own. This work was crucial to her having something real and original to say in her essays and she was asked about it in great detail in her interviews. SHe is also a serious singer and had been on TV and recorded dvd's, so was interested in Cam choruses.
First, to apply, you have to meet strict grade requirements. If you are a foreign applicant, from a different system, this adds a bit of flexibility, but not much. In fact, standards from admission from abroad are MORE stringent.
Second, if they are interested in you, they may ask for very personal essays that require research and an expression of the applicant's own thoughts. Parents are instructed to let it be a pure product of the applicant, with no writing help. I was very impressed with this, as it was a growth experience for my daughter.
Third, they can invite you for an interview on campus. Beyond probing what you know until they stump you, this is to check if you have the "right attitude", that you "shine" yet know your limits. It was an exercise in humility for my D.
Fourth, they can ask you to take tests in your topic, though this was not required of my daughter.
Fifth, if they make you an offer, it is conditional. They set precise overall grade requirements (or class ranking) - my daughter had to get very high grades on her BAC exams (in her French high school), in some cases for specific disciplines; my daughter had to get A+ in honors math.
For this final hurdle, we formulated a strategy. We looked at her strengths and got a tutor to prepare for her BAC exams (they demand a specific style of answer, which was what we prepared for - not content, but style), in a sense knowing what she could ace and how it was weighted to affect the overall score. We avoided emphasis on subjectively graded subjects, such as philosophy, and concentrated on those over which we had more "control".
As one can imagine, it was a long and stressful process right up to last July, when her BAC results were posted at school. We were focused on the process for the last 2 years, though from age 12, my D had expressed interest in going to Oxford and had read a book about how to get into Oxbridge. As it turned out, she visited Oxford at 17 and didn't like as much as Cambridge.
I hope this helps.