The GM should design cults to fit his purposes, but below a general guide to design is given.
The worshippers of the Forest Man do not have cults as such, being instead forming loose communities, based around priests and priestesses. There are rarely more than a few dozen of these, even in larger communities - more often only a half-dozen, or even a single hermit. They do not have cult secrets or special magics: the gifts of the Forest Man are granted at a ceremony overseen by a priest or priestess, but their form is decided between the worshipper and the Forest Man.
Cults range in size from a handful of individuals to hundreds, depending on the size of the community, they draw on. All cults are based on a church or shrine, although some cults (such those based around military companies) may have a portable shrine that they take away from their home base when on the road. A cult always has a head priest - if it is large, it will have more priests and initiates. As a general rule, about 5% of a cult will be Adepts (who are taught the cult's secrets), and about 10% Initiates, but cults of less than 50 members usually do not have such ranks.
In addition to cult spells, many cults have secret skills/powers, which they teach members. Treat these as talents and choose (or create) talents appropriate to the cult (for example combat talents or fighting stunts for the Horned Man, talents related to the sea - the ability to hold your breath for 5 minutes, for example - for the Woman of Sighs, and so on) Cult knowledge can be generated thus:
The cult will have access to 1d6+2 basic skills. These will be skills associated with the cult figure, or the community in which it is based (so a cult based in a fishing village on Dendrith might teach PS: Fisherman, AK: Coastal waters of Dendrith and Swimming, for example).
In addition, the cult may have 0- 3 secret cult skills (d6-3) with a chance for more in larger cults. These could be unusual skills, talents or powers such as fighting stunts. For both basic skills and cult secrets, add +1 to the die roll if the cult is more than 25 members, +2 if more than 50, +3 if more than 100, etc.
And of course, cults teach magic. The mechanism is detailed under the magic section. To find which spells the cult has, it has a chance of having any given spell equal to the number of cult members minus the real points of the spell.
These cults have the same general structure as the cults of the Twelve - however they will also have a Patron, who in some cases actually replaces the god. The spells and secrets he can teach are much broader than those of the Twelve, but they have a price. The areas in which each patron can grant spells and knowledge is detailed in the section on magic.