Technically what we are doing is installing an Addon Domain to your account. Basic hosting allows for 1 domain and therefore website. Other packages allow more, you can upgrade if you need to. If you are unsure about what this actually means, read this.
Log into your domain management panel: cpanel.yourdomain.com.au
Your login details are in the welcome email you should have received. If you can't find it, lodge a support ticket and it will be resent
Don't be intimidated by all the offerings in cpanel, it can be overwhelming. Also be careful you could break the internet...well, maybe just your website...no kidding.
You are looking for the Domains section and particularly the Addon Domains icon - click it
Next fill in the domain name and the subdomain/FTP username and document root should fill automatically.
The document root is where the physical files are stored in your account. Give it a meaningful name to distinguish the new domain from your existing domain (which lives in the public_html directory). It should default to the new domain name as in the example below. This means that when in the file manager it is really obvious where the files for 123host.net.au are located.
Add a password or use the password generator to create a formidable password.
Then click Add Domain
You should now see something like this: It means that the files for 123host.net.au are located in /public_html/123host.net.au
Don't worry about redirection. If you are redirecting, you should use a parked domain rather than an addon domain.
That's it. Try out the domain in your browser and you should see an empty file structure just waiting for you to install wordpress or something else.
Stuck? Lost? Adding to your confusion? Lodge a support ticket