The digital SAT is taken at a test centre on a device, either the student's own or one provided. That changes what preparation for the day means.
Before the day
The testing application has to be installed and the exam ticket downloaded in advance. Doing this at the centre is not an option.
The device needs to be charged, and a charger brought regardless. Centres do not guarantee power outlets.
Identification
A valid photo identification document is required, and the name on it must match the name on the registration.
This is the single most common reason candidates are turned away, and it is entirely avoidable: check the registration against the document weeks in advance, and correct it through the registration system if it differs.
What to bring
Identification, the exam ticket, the charged device and its charger, an acceptable calculator if you prefer your own, and something to drink and eat for the break.
A calculator is built into the application, and a separate approved one is permitted. Practising with whichever you intend to use matters more than which you choose.
What happens
Check in, device setup, then two sections each in two modules, with a break between the sections.
The application shows a countdown and allows it to be hidden. Whether that helps is personal, and the setting should be tried during practice rather than discovered on the day.
The travel rehearsal
Going to the centre once beforehand, at the same time of day, removes a whole category of exam morning problems. Traffic and an unfamiliar building are not the things to be solving an hour before the exam.
The principle
Nothing should be encountered for the first time on exam day. Everything on this page can be settled in advance, and everything that is not settled becomes a decision made under pressure.