(1) Dry them. If it's sunny outside, place them in a sunny spot outdoors. When they are dry, then lightly brush off the excess dirt.
(2) Cure them. Find a cool, dark, dry, but well ventilated place where they can "cure" for a couple of weeks.
(3) Dust them. To prevent rot, you can dust them with an anti-fungal powder. (I admit to skipping this step often, but then I live in a dry climate.)
(4) Wrap them. Wrap each iris rhizomes in a piece of newspaper; place these in a box. I use stackable open weave plastic bins.
(5) Store them. Again, you'll want a cool, dry place.
(6) Check them. As you have probably gathered by now, rhizomes are subject to fungus and rot. Every so often, check your stored rhizomes. They should feel firm. Discard those that seem soft or mushy so those don't spoil the whole lot.