Securing a project involves two steps: setting a password and locking the project. The Editor allows you to manage passwords for the current project. Any project, be it the current project or a nested project, can be locked.

Locking a project

Open the view that contains the target project. If the target project is the current project then open the project view (select View → Project View from the menu bar).
Select the Pointer Tool.
Select the target project from the contained features. If the target project is the current project then click on the design surface.
In the Properties panel, check the Protect property value if it is not already checked.
If a password has been set for this project but you have not yet supplied it, you will now be prompted to enter the password. The lock operation will succeed only if you enter the correct password. Failure to do this will leave the Protect property value unchecked.

Setting the current project's password

Select Project → Set Password... from the menu bar.
The Set Password dialogue will open. You will be prompted to enter both a new password and a confirmation of this new password. If a password has been set for the current project, you will be prompted to enter this as well.

Clearing the current project's existing password

Select Project → Set Password... from the menu bar.
The Set Password dialogue will open. You will be prompted to enter both a new password and a confirmation of this new password and you should leave both fields blank. If a password has been set for the current project, you will be prompted to enter this as well.