Much depends on the are in which you live. If you have the option of choosing between several schools, you need to visit the schools. Don't rely exclusively on hear-say and other people's opinions. Very often those opinions were formed years ago and may even be based on gossip. Go and visit the schools. See for yourself.
Depending on the market where you live, places may be at a premium. Begin your school selection process as early as you can.
Each private school is unique. So expect differences in the admissions procedures. While every school will require at least one interview, a formal application and testing, precisely how each individual school goes about each part of the admissions process is something the school decides. In other words there is nothing uniform. Expect differences. Some subtle. Some rather obvious.
The school will want to meet you and your child. What are they looking for? Pretty much the same things that you are looking for. They want to make sure that you and your child will fit in. Wait a minute! What do you have to do with this? You are not going to the school. Your child is. That's the point: the school wants to make sure that you are going to be a willing and able partner with the school in your child's education. It needs to know that you are in agreement with the school's methodology and philosophy. It needs to know that you child will flourish in the school's environment.
Depending on the school, you will be interviewed by a teacher or an admissions staffer or the head of school. In most cases you will be interviewed by all three.
Applications for pre-school and primary school are not nearly as extensive as those found at the high school level. There are no teacher recommendations and transcripts to worry about. The school is probably going to insist on some commitment to volunteering a minimum number of hours to help the school.
Admissions are generally on a rolling basis. That means that once all places are filled, the rest of the applicant pool is put on a waiting list. Make sure you adhere to any deadlines the school sets forth.
Most of the time pre-schools are interested in making sure that 4 and 5 year olds are potty-trained, socialized and ready for school. Schools will try to determine whether the child will be able to keep pace with the school's academic program. The technical term for this testing is developmental assessment. Each school administers its own testing routines according it its established protocols. There is no standarized test for primary school admissions.
Should you hire a consultant to prep your pre-schooler for the admissions testing and interviews? In my opinion this is unnecessary, though I am well aware that pre-school children are 'coached' or 'prepped' in major metropolitan areas such as New York. If you have done a good job of parenting, your child will under normal circumstances reflect that love and attention. So, relax and let your child be herself.
If the school decides not to offer your child a place, you have no recourse. The decision is generally final. Move on and find another school which will accept your child. If the school puts your child on a waiting list, you would be well-advised to have Plan B ready just in case the place never materializes.