I have hired a bunch of senior developers for some of my clients as part of my CTO-as-a-Service consultancy. I have also hired many senior developers in my past lives in large corporations.
I think that the main thing that I need to ensure is that the new developer does not experience a sense of regret and frustration when they walk in the door. I remember the things that have frustrated me in the past, and I make sure that these situations are not repeated with the new developer.
Here is what I try to have set up on the day that they join:
1) New laptop with enough power that a heavy-duty developer needs.
2) All accounts have been set up. Nothing more frustrating than having the developer sit around for a few days, waiting for access to email and Github.
3) All of the software has been licensed and (maybe) pre-installed. This includes all third-party frameworks and tools that require subscriptions.
4) Up-to-date Wiki and Jira (or whatever project-management software the company is using). Make sure that the architecture and system documentation is up to date.
5) Clear tasks defined for the first few weeks. Maybe there is a small feature that the app needs right away? Give it to the new dev to get them warmed up to the codebase.
6) All HR and Payroll-related items are done. If the person needs a company credit card, the card (or the application for the card) is waiting for them.
7) Proper introductions to the senior team. Does everyone know that the senior developer is joining? Do they already know how the senior developer aligns to the success of the company? If the senior developer is aligned to a certain business unit, do the people in that business unit know what the senior developer will be working on?
8) Make sure that there are people around to answer questions. Especially if the codebase has tricky parts that are difficult to understand. Make sure that all important architecture decisions have been memorialized on the Wiki.
There is nothing more satisfying to the new developer than hearing someone say “XYZ really hit the ground running, and has made an immediate impact”. Do everything you can to make sure that the new developer gets to hear that sentiment expressed by your senior staff.