An onboarding plan turns onboarding into a process rather than luck. It divides the time into stages, for example the first day, the first week, the first month, and the first 90 days. For each period, it states what the new hire should be able to do, which courses to take, and who to meet.
The plan serves both the new employee and the manager. The new hire can see what is expected. The manager can see when to follow up.
Many items in the plan are the same for everyone in a role. You can gather those in a program and assign it automatically, so the manager only needs to add what is specific to the individual.