I just received my order of bare root Iris so I took some pictures.
First find a pot that is the correct height so if the rhizome is near the top of the pot it won't be submerged.
Put some soil or planting media in the pot. Use clay or loam or gravel from the ground (not soil from your flower or other growing beds that might be full of chemicals or residual fertilizers.)
Note: Do Not Use regular Potting or Planting soil. It is too rich in woody material and it will float out of the pot and into your pond.
You can also purchase growing media for aquatic plants or buy a bag of pea gravel or other small stones and use that to weigh the plant down.
I use soil from our property plus pea gravel from a local garden supply center. The pot needs to be heavy enough to not float away.
Image 1: I put a bit of soil and gravel in the bottom with a pond fertilizer tablet.
Image 2: Put the rhizome in the pot and adjust the roots so they are pointing down.
Image 3: Add the rest of the dirt covering the rhizome but leaving the green buds and stem uncovered.