First, get your Numbers document up. Go to File and slide down to Set Password…
Next, enter a password, enter it again, and enter a hint. The hint is there in case you forget the password. Warning: I can’t get your document open if you forget the password. Give yourself a good hint.
If you check the box to remember the password in the Keychain you will never have to enter it again, as long as you’re logged onto the Mac with the same username and password. That reduces the value of having a password on the document so my advice is to NOT check that box.
That’s all there is to it. From then on, whenever someone tries to open that document he’ll get a box asking him to enter the password. Even if he’s a she. Get it right and the document opens right up. Otherwise, the document remains closed. Don’t check that “Remember password in my keychain” this time either.
That was easy.