Read the fax in East Foyer, then copy the entire letter cluster containing each needed letter from the Nursery chalkboard. The other clusters can stay blank. Spaces are ignored.
The reference photo shows the layout. Its letters may differ from your match. Count the requested letter too.
Choose the protocol printed by your fax machine.
Saved on this device; shared with the full-page helper. Screenshots: COD Zombies Guides and r/CODZombies, linked in the map guide.