First rule is plenty of room - iris rhizomes "travel'. Use good garden soil - not too heavy or light.
Lightly sprinkle bulb food, bone meal, or superphosphate on the lower half of the soil & mix it in. Pat soil down, add more, pat down --- place rhizome and add soil around it, not to cover completely.

Thanks irisgal. Another couple of questions, if you don't mind. Are these full sun plants? And looking far ahead, how do you care for irises over the winter?

Yes, definitely, irises are full sun plants. There is no need for any special winter care for irises, except, perhaps, if your area is prone to freeze-thaw cycles, then mulch the irises for the first winter. Apply this mulch late fall, then when spring comes, remove it. Once they are well established, there will be less possibility of the rhizomes being lifted out of the soil by frost.

Another thing to do is late fall, cut the leaves down to about 6 or 8 inches tall and remove any dead leaves.

Good luck!