Use the Security tab of Tools | Options.
From Word Help:
*****************
Security Use strong passwords that combine upper- and lowercase letters,
numbers, and symbols. Weak passwords don't mix these elements. Strong
password: Y6dh!et5. Weak password: House27. Use a strong password that you
can remember so that you don't have to write it down.
1. Open the file.
2. On the Tools menu, click Options, and then click Security.
3. Do one of the following:
Create a password to open
1. In the Password to open box, type a password, and then click OK.
2. In the Reenter password to open box, type the password again, and
then click OK.
Create a password to modify
1. In the Password to modify box, type a password, and then click OK.
2. In the Reenter password to modify box, type the password again, and
then click OK.
*****************
