bumblebee, I was going to post this same question. What I do is use a mix of sand
sphagnum moss, large bark chunks, and ( gasp, horror, oh no, say he didn't say it)
my old bonsai soil from repots, and deadness. Next excavate the hole, a piece of stone, etc. is supposed to be good for the bottom of the hole, I don't think it's a big deal either way, you'll work on nebari, after field growing.
Put a layer of the mix(soil) over the root barrier/ the bottom of your new hole, in with your tree, no deeper than it was before, now you massage the outer layer of the root ball, carefully mind you if it's the hot season, or the tree has been stressed recently. Once you have situated your tree, I like to put in stakes on each side of the tree, to be used for supports for a crossbrace, which you can tie the tree to to keep it from breaking new fine roots, untill it has settled in, then I remove them, to let the wind stress the tree. The time frame here for new growth to anchor the tree could be a couple of years. This is the place I say finish backfilling around the rootball with the same soil mix we mixed up at the start, mounded a little, because I let it settle itself in with watering, one good reason for the supports early on. Some times I use a bark mulch, sometimes not, depending on the water retaing ability of the soil at the time, location, in wind, and sun exposure.
Water like the rest of your trees, when they need it, remember you need to know the soil in the original rootball, and your fill mix, new roots can't dry or they die, and conversely, wet roots rot and die, pay attention to the soil in both zones of your tree.
Ah, you thought I forgot a part, I am sure I have. Someone will fill those slots sooner than later, I am sure, but I did think of something else, I dig the hole about 12" in diameter bigger than the root ball, and use the outer edge of that as a rootpruning guide for fine root build up, and tree removal.
Thats what I do, hope it helps
Bill