The character write instructions or other information on parchment, paper, or any suitable writing material. The illusory script appears to be some form of foreign or magic writing. Only the person (or people) designated by the character at the time of the casting are able to read the writing; it’s completely unintelligible to any other character, although an illusionist recognizes it as illusory script.
Any unauthorized creature attempting to read the script triggers a potent illusory effect and must make a saving throw. A successful saving throw means the creature can look away with only a mild sense of disorientation. Failure means the creature is subject to a suggestion implanted in the script by the character at the time the illusory script spell was cast. The suggestion lasts only 30 minutes. Typical suggestions include "Close the book and leave," "Forget the existence of the book," and so forth. If successfully dispelled by dispel magic, the illusory script and its secret message disappear. The hidden message can be read by a combination of the true seeing spell with the read magic or comprehend languages spell.
The casting time depends on how long a message the character wishes to write, but it is always at least 1 minute.