There are a couple of ways to do it. First unsulate under the dlab with blue foam or foil-faced poly foam, the kind that is closed-cell so it doesn't soak water. This way the heat stays in the slab to go up instead of into the ground.
One way is to put the tubing down in the pattern with 4x4 hardware wire cloth over it and secure the tubing to the wire with soft plastic ties. Fill the system to check for leaks and weight the tubing. Pressurize it and watch for pressure loss and leaks. Pour the concrete if satisfactory.
Another way is to make a 2-inch pour over staked ground. Secure the tubing to the top with the wire cloth to the stakes . fill and check. then pour the final layer.
Some manufacturers make plastic forms to insert and secure the tubing. Then pressurize and check. Then pour over the forms.