Create a file named .htaccess in the directory you want to protect. For example, assuming that the directory is named /var/www/secret:

AuthUserFile "/var/www/secret/.htpasswd"
AuthGroupFile /dev/null
AuthName "My Secret Directory"
AuthType Basic
Require valid-user

Give one or more users access to the directory. For example, to create a user named "admin":

htpasswd -c /var/www/secret/.htpasswd admin

(The "-c" assumes that .htpasswd doesn't exist yet.)

htpasswd will prompt you for the password.