If you're asking about, say, Iris reticulata, and your soil has 'set' for the winter - plant them in pots and keep them on the cool side - around 40-45F. They don't need to be put in the dark: just a shady windowsill (away from any radiators) until they come into bud and can go into more sun.

Keep the compost moist - but not soggy. Really well-drained, so no standing in a saucer of water.

Reticulatas seem to have different flowering times depending on their colouring/breeding so you'll need to be patient.

If you find any coming up with yellow and droopy leaves - investigate promptly. It could be a faulty bulb.

For the rhizomes - if they are early dwarf varieties that you hope will be out with the daffodils - pot them up, too, using the newest parts of the rhizome where the leaf fans are.

Ideally, these should have been planted in the fall :-) Most bulbs and rhizomes do not benefit from much storage so potting them up ASAP as indicated in the previous post is advisable. Most bulbous iris tend to be spring bloomers, some very early, so really need that soil medium to develop their roots and chill time to be able to flower this coming season - you may have missed it. Most rhizomatous iris are later spring/early summer bloomers but often need time to establish well before putting on their show. Regardless, both types will appreciate being potted up, if not planted out, as soon as possible to preserve the viability of the bulb/root stock.

I would have planted them sooner but I just received them in a trade. :-) I did plant them up last night and set them in the basement. Hopefully they'll be ok... I continue to check on them throughout the winter.