I was fortunate enough when I was going to school to be able to visit and walk the halls, meet my teachers and explore before going to the school. And I was thrilled that my daughter’s school had that same type of open house. If your schools have that event, take advantage of it. At the open house Teachers held meet and greets, showed kids the classrooms and made them comfortable. It also allowed for kids to start to form companionships.
If this is not available, and you want to make your child comfortable, you can arrange a visit to any school so your child will have an idea of how classrooms work and who the teachers are. All it takes is a phone call request to the school. You can even visit a school on down time and wander the halls and look in classrooms. Often if there is a teacher available they can walk you and your child through the process, location and what they can expect. This way your child really becomes comfortable in his/her new surroundings.
Another way is to have mock school at home,. Get some old school books from a used book store or purchase home school work books( which will give your child a heads up on what is upcoming that he/she will learn and take the frustration out if they don’t get a concept) and set a time to “ play school”.
They think it is fun and actually take the scheduling and expectations and scariness out.

Make getting their school supplies an event. If your school publishes a list of what you need to buy, print it out and allow them to choose items that they feel comfortable with and explain to them how much money they have to spend.
Many schools now have dress codes, so those are often flexible but not too liberal. Individuality is often s comfort factor as well so choose items that can choose daily that wont interfere with that dress code rule. Small piece of new jewelry, a new watch, fancy underwear, fun socks etc. Something special for them as well as piece of identity.

Be a supportive parent. Today there are many other things that affect going to school and getting ready for school. Ask questions and be supportive. Do not poopoo apprehension, fear or other items that may be a point of having a bad experience.

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The most important thing is to be sure they already know how to read before they go to school. Nothing else matters. If they cannot read, and read quickly, your kids are doomed to stay behind those who can.

Let them stay at an Aunt's/Uncle's house a few nights so they get used to being away from you and home. I assume you take them to the park and let them play with other kids so they should be used to that. Then when I went to Kindergarten the Parents just left the kids there and waited in the hallway for them all to stop crying. There was always the kid who wet his/her pants. I think you have to get them used to seperation for this. All this new stuff with games that no one loses and all is wrong so that no kid ever has bad feelings. This is why they are unprepared for the real World. There are too many things we now prepare for that we need to just let them live through.