Simply log in to your account and go to 'Register New Domains' - follow the onscreen prompts that will guide you through submitting your domain application.
After you submit your order keep an eye on your emails as in some extensions we may have needed to contact you for further information, documentation or action on your part to move the registration forward. If you want to know if there is anything special about registering a particular domain extension that you have never registered before please contact us.
If you are wondering how long things will take check out our How many days until my domain is registered? FAQ entry.