Prepare access before the start date, not on it. Their email account on your domain, their own logins to each tool rather than shared credentials, a seat in the password manager with access to specific items, and whatever hardware they need already configured. Somebody spending their first two days waiting for access learns that the business is disorganised, which is the first impression you cannot retake.
Handle the compliance paperwork through your payroll platform rather than on paper, because it will collect the tax forms, verify eligibility to work, and handle state new hire reporting within the required window. Those are obligations with deadlines rather than administrative preferences, and a system that tracks them removes an entire category of risk.
Then give them something written for the three or four things they will do most often. Not a handbook, which nobody reads, but a short document per task describing how it is actually done. This is the piece that determines whether they are productive in days or weeks, and writing it is usually the moment you discover that a process you thought existed only lives in your head.
Build the offboarding list on day one while you have the full picture. Every account they were given access to, recorded as you grant it. When they eventually leave, whether in six months or six years, removal is a checklist rather than an attempt to remember.
Ask the person you just onboarded what was confusing, in their first fortnight while they still remember. They are the only person who can see the gaps, because you have been inside the business too long to notice what is unstated. Whatever they name goes into the documentation immediately, which means each hire improves the process for the next one rather than each hire rediscovering the same problems.
Schedule the first week rather than leaving it open, because a new person with unstructured time and no context will either wait or guess. A simple plan naming what they will do each day, who they will speak to, and what they should be able to do by Friday removes the anxiety on their side and the improvisation on yours. It also surfaces immediately whether the access you prepared actually works.