From “Herb Gardening for the Midwest” by Debra Knapke & Laura Peters:
Fill a pot or planting tray to about 1” from the top with perlite, vermiculite or seeding soil mix. Moisten the soil and let the excess water drain away. Lay the rhizome pieces flat on the top of the soil and almost cover them with more soil mix. If you leave a small bit of the top exposed to the light it will encourage the shoots to sprout. The soil mix does not have to be kept wet, but you should moisten it when it dries out to avoid having your rhizome rot. Once your cuttings have established themselves they can be potted individually and grow on in the same manner as the stem cuttings (see p. 28).