The way I do it here (not Texas but hard clay subsoil) is I start by edging and peeling up all the sod in a big circle about 3-4 feet wide. Usually there is at least 1"-3" layer of clay attached to the bottom.
I don't dig any deeper than that, but thoroughly break up the clay soil under neath to a garden fork depth (NOT shovel) which achieves the rough sides mentioned above and avoids the smooth.
I then dump a layer of unscreened mostly finished compost (ususally contains big pieces of sticks, crab claws, seashells, corn cobs, some leaves and straw as well as resident earthworms and other denizens) and small (pea sized but sharp) gravel and composted bark chip mulch plus a layer of top soil collected from under leafpiles in another part of the yard. I usually add some rock phosphate and green sand as well as dolomitic lime depending on the tree. -- fork that in.
Usually, this is enough loose soil to plant the tree slightly raised above the grade.
I will be laying all the sod grass side side down except in the middle where I'll be planting the tree. Here, I chop the sod into small chunks and put them in the bottom of the tree planting hole. Then plant the tree on top.
I end up with clay around the perimeter of the circle and mounded soil in the middle. I put the sod grass side down with all the clay on the top. The sod will help to hold the mound while I water thoroughly. If the water tends to pool and not soak into the clay in the circle, I'll cut breaks in the surrounding sod to help drain it away, sometimes creating a mini V trench with my edger or shovel.
Then put another layer of compost on the clay underside of the sod which is on the top now. Cover the entire circle with wet cardboard and/or several layers of newsprint (newsprint where ground is contoured) to smother the grass and weeds, and then cover them with decorative mulch.
In less than a week the whole area will be full of earthworms.
I water thoroughly every other day during the first week, twice a week for the next two weeks, then once a week for the next month and the rest of the season if it doesn't rain.