Apache comes with a command line utility called "htpasswd". This utility will generate a username and password that you can use to authenticate against using a .htaccess file. Just run the utility like so:

  htpasswd -c /path/to/your/password/directory/and-your-password-filename jane_doe_username

Keep in mind that this will not create a user on the system. It will just create a password and associate it with a string, that is the username you'll use to authenticate in your request.

Then you'll need to add a .htaccess file in the directory that you want to protect, and in that file place the following code.

AuthType Basic
AuthName "Restricted Directory"
AuthUserFile /path/to/your/password/directory/and-your-password-filename
Require user jane_doe_username

Remember that the /path/to/your/password/directory will need to be owned by Apache. on Ubuntu, it's by default the www-data user. Change ownership like so:

 chown -R www-data:www-data directory