First of all, when writing a message, you should put it in an eye-catching place so that you can see it at a glance. If you put it on the shelf, your mother probably won't see your message.

Secondly, you should write clearly who this message is for. If it's for your mother, then the beginning is to write mother first. As follows.

Mom, I'm going out for a while today. Go to the library to read books with people at two o'clock in the afternoon. I will go home at five o'clock in the evening. Don't worry about me. -Your so-and-so

Write things clearly, how long to go out and when to come back, all of which should be written clearly. The date should also be marked.