More info to follow, but the key steps are:
Keep those roots wet!
If you haven’t planted your grafted tree and haven’t decided where to put it (and this is worth bearing in mind as it’ll grow fairly big, the trees are grafted onto M106 rootstocks, more info on that to follow) then put it in a fairly big pot and you can keep it in there for a year, sometimes even 2!
But if you haven’t got a pot big enough for a tree yet, then just make sure that it’s roots are wet – stick them in a bucket of water, and run out and grab a pot as soon as you can so you can house your there!