It's a simple two step process
In your .htaccess put
AuthType Basic
AuthName "restricted area"
AuthUserFile /path/to/the/directory/you/are/protecting/.htpasswd
require valid-user
use http://www.htaccesstools.com/htpasswd-generator/ or command line to generate password
and put it in the .htpasswd
Note 1: If you are using cPanel you should configure in the security section "Password Protect Directories"
EDIT: If this didn't work then propably you need to do a AllowOverride All
to the directory of the .htaccess (or atleast to previous ones) in http.conf followed by a apache restart
<Directory /path/to/the/directory/of/htaccess>
Options Indexes FollowSymLinks MultiViews
AllowOverride All
</Directory>
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