Set your iris in place, spread out roots and cover with soil. The rhizomes should be even with the soil surface in heavy soils and just slightly lower in sandy soils.
Water to help remove air pockets and get your iris off to a growing start.
A bit more information: Discovered iris borer while transplanting? You can help reduce future problems with a bit of fall cleanup. The adult borer is a day flying moth that lays its eggs in the leaf debris. By removing all the dead iris leaves in fall, you will break the life cycle and usually eliminate the problem.