How are Doll Knights summoned?

This comment asked about certain things that may not be immediately obvious so far in the series, namely how exactly are the Doll Knights summoned.  In light of this, I'm going to see if I can explain it a bit more clearly.

Basically, there two requirements to summon each Doll Knight:

1. Each Doll Knight's Calling Bracelet must be on the person that will be protected.  This is why Yae could not summon Doll Isamu until Dai found his Calling Bracelet (and Scarecrow took it and put it on).
2. Each Doll Knight's master control trinket where the summoning words are uttered must be equipped.  (This requirement is altered starting in episode 28.)

Let's go to each Doll Knight for specifics.

Doll Licca

Doll Licca's Calling Bracelet is currently being worn by Rika.  Her control trinket, the Calling Pendant, is worn by Nanae (Rika's grandmother).

Doll Isamu

Doll Isamu's Calling Bracelet is currently being worn by Scarecrow.  His control trinket, the Calling Watch, is worn by Yae (Nanae's twin sister). (It's called a watch because it looks like one of the old-fashioned windup watches that is on a chain.)

Doll Izumi

(This is behind a cut because there's a spoiler about 10 episodes ahead.) 

Doll Izumi's Calling Bracelet is currently in Catherine's possession.  Catherine actually hides the bracelet in her clothes to prevent drawing too much attention to herself (you'll see it fall out of her clothing later).  Her control trinket, the Calling Choker, is worn on Catherine's neck (you might have seen it).