"You buy trees in containers now.
Some of them may have only just been put into that plastic pot and may not have much of a root system. While you can plant them at once, there are the conservatives who believe that there are two times for planting, late autumn, say November, and then spring. Such people dig a hole for tree and tainer, and drop it in until they come to the right time.
It's not hard to find a simple diagram on the size of hole you should dig, the way you should break up the soil not too fine, for them it may cake when you water it. Big rule the collar of the tree, i.e., the level at which the earth has held it, should be the same in its new home.
Above all, water it. And watch it for more watering. Staked and ties depend on the size of tree.
As to roots, you must see to it that they are well spread out, as you will grasp from this poetical piece from Thomas Hardy's The Woodlanders, quoted here once before.
Henry Winterbourne "had a marvellous power of making trees grow. Although he would seem to shovel in the earth quite carelessly, there was a sort of sympathy between himself, the fir, Oak or beech that he was operating on so that the roots took hold of the soil in a few days Marty, who turned her hand to anything, was usually the one who performed the part of keeping the trees in a perpendicular position whilst he threw in the mould . . . The holes were already dug, and they set to work.
Winterbourne's fingers were endowed with a gentle conjuror's touch in spreading the roots of each little tree, resulting in a sort of caress under which the delicate fibres all laid themselves out in their proper directions for growth. He put most of these roots towards the south west, for, he said, in forty years time when some great gale was blowing from that quarter, the trees will require the strongest hold fast on that side to stand against it and not fall.
How they sigh directly we put'em upright, though while they are lying down they don't sigh at all said Marty. `Do they?' said Giles, I've never noticed it. She erected one of the young pines into its hole, and held up her finger the soft musical breathing instantly set in, which was not to cease night and day till the grown tree should be felled probably long after the two planters had been felled likewise. `It seemed to me' the girl continued as if they sigh because they are very sorry to begin life in earnest just as we be. `Just as we be?' He looked critically at her. `You ought not to feel like that, Marty'." But this was a Thomas Hardy novel.