To set a password on a Microsoft PowerPoint document, follow these steps:

1. Open the PowerPoint presentation that you want to protect with a password.
2. Click on “File” in the top-left corner of the screen.
3. Select “Info” from the menu that appears.
4. Click on “Protect Presentation” and then “Encrypt with Password.”
5. Type a password in the “Encrypt Document” dialog box and then click “OK.”
6. Re-enter the password to confirm it and then click “OK” again.
7. Save the document and the password protection will be applied.

Please note that once you set the password, you will need it to open the document. Also, you should store your password in a secure place as there is no way to recover the password if you forget it.