How to Set Permissions on Your MacBook

By Mark L. Chambers from MacBook All-in-One For Dummies, 2nd Edition

You can set permissions on your MacBook to increase the security of your documents and to prevent yourself from inadvertently deleting files. To set permissions, follow these steps:

Click the item to select it, press Command+I (or choose Finder→File), and then choose the Get Info menu item.

Alternatively, you can right-click the item and choose Get Info instead. Either way, Mac OS X displays the Info dialog.
Click the right-facing arrow next to the Sharing & Permissions heading to expand it.
To change your own permissions on the item, click the Privilege pop-up menu next to your name — handily marked “(Me)” as well — and choose a new Ownership permissions level.

This is likely set to Read & Write, and it’s a good idea to leave it alone. If you’re the file’s owner, you’re likely not a security risk.

Never choose an access level for yourself other than Read & Write without being absolutely sure of what you’re doing, because you can potentially prevent yourself from accessing or deleting the file in the future!

For example, if you simply want to lock an item to prevent changes being made, don’t set your Ownership permission to Read Only. (Instead, select the Locked check box in the General section of the Info dialog instead . . . you can easily clear the Locked check box later to make changes to the item.)
To change permissions for someone else or a group, click the Privilege value for that user or group and then choose the appropriate value from the pop-up menu.

Assigning permissions for an entire group is a good idea for limiting specific files and folders to only Administrator access. (Note, however, that Lion reserves the group name wheel for internal tasks, so never alter any permissions for the wheel group.)
If necessary, set the permission for the Everyone pop-up menu (otherwise known as “I’m going to lump everyone else into this category”).

If a user isn’t the owner of an item and doesn’t fit into any group that you’ve selected, this access permission setting for this file applies to that user.

Need to apply the same permissions to all the contents of a folder — including subfolders within it? If you selected a folder, you can click the Action button at the bottom of the Info dialog (which carries a gear icon) and choose Apply To Enclosed Items from the pop-up menu that appears.

After you confirm the action, Lion automatically changes the permissions for all the items contained in the folder to the same settings.

Generally, it’s a good idea not to override the permissions for all the items in a folder, so use the Apply to Enclosed Items function only when necessary.
After all the permissions are correct, click the Close button to save your changes and return to your friendly Finder.

If a specific user or group doesn’t appear already in the Privilege list, click the Add button (bearing the plus sign) and you can add a specific privilege level for that user or group. You can also delete a privilege level: Click the desired entry to select it and click the Delete button (which bears a minus sign).