Let him land before you ask him anything
The journey, the new smells, the strange horses calling — arrival day is a lot for any horse, so make the first days deliberately boring. Put him in a stable or pen where he can see and hear the others without being turned out among them, with hay and water already in place, and keep handling simple: lead him, groom him, pick out his feet, let him look around the yard at quiet times. Many yards also keep a new arrival apart from the herd for a period before mixing, which protects the group if he has brought anything with him. Resist riding on day one; a horse that has had two or three days to learn the layout, the noises and you is a far safer prospect than one ridden straight off the lorry.
Copy his old routine first, then blend it into yours
Sudden change is the enemy, especially in feed. Keep his forage and hard feed as close to what he was on as you can, get the exact amounts from his previous owner, and change anything gradually over a week or more — gut bacteria adjust slowly, and abrupt feed changes are a classic route to colic. Fit the yard's routine around him at first: if he is used to being turned out early and fed twice, keep that pattern, then shift timings gradually towards the yard's. Horses read routine as safety; consistent feed times, turnout times and handling order settle a new horse faster than any amount of fussing.
Introductions over the fence before in the field
Let him meet his future field companions with a fence between them first — mutual sniffing, squealing and striking at a fence is normal and safe — then turn him out with one calm, steady companion before joining the whole group, in a field with room to get away and no tight corners to be trapped in. Stay and watch the first turnout properly rather than glancing from the gate: most herd sorting is noise and galloping that settles within the hour, but a horse being relentlessly chased or one that will not settle needs bringing back in and a slower plan. Expect the full social settling to take a couple of weeks, not a couple of days.
Learn his normal and write it down
While he settles, you are collecting his baseline: how much hay he leaves, what his droppings look like, where he chooses to rest, whether he drinks well, how he behaves at feed time. That baseline is what tells you later whether something is wrong, and it is also what anyone else handling him needs to know. Note his quirks honestly — bargy at the gate, nervous of the hose, perfect to catch — because the next person leading him deserves the real picture, not a surprise.
The settling goes smoother when his information travels with him, which is the point of KinsLe's Transferrable Profiles: his profile moves to you with his feeding, routine, history and quirks intact, so the routine he is used to is in front of you from the first morning rather than pieced together from phone calls and guesswork.