Assessment day is a sequence of small rooms with strict clocks. You will be told when to stop. You will be told not to talk about the items. You will feel that everyone else knows a secret. They do not. They are also watching a square.
The shape of the day
It varies by operator. A common pattern is: arrive, identification, briefing, then a battery of psychometric tests for a couple of hours. Some centres put the interview on the same day. Some put the medical there too. Some split them. Read your invite like a working timetable.
The psychometric block is the part people mean: vigilance, search or scene perception, concentration, rules, perhaps dual-task work. You get instructions for each one. Those instructions beat every article you have read, including this one.
What it feels like
The first test is novelty. The third is fatigue. Useful preparation is having already been bored on purpose, so fatigue is familiar.
You will want to replay the last item during the next instruction. Do not. The last item is gone. The next instruction is the only thing that can still change your score.
Interview and medical
If there is an interview, they are not looking for a speech about childhood train sets. They want to know whether you notice risk, whether you stop when you should, and whether you can talk about a mistake without swagger or collapse. Short true stories beat long invented ones.
The medical is a medical. Sleep. Do not experiment with a new energy drink that morning. If you take something the doctor should know about, say so. Hiding it is a worse idea than the thing you are hiding.
The night before
Do not cram. Cramming is for facts. These tests are for habits. Sleep is a habit. So is breakfast you already know sits well.
If you have practised the formats, the day is still long, but it is not alien. That is the whole advantage. Not a secret pass mark. Just fewer surprises.