In order to use a domain registered somewhere else with your SiteGround web hosting account, please follow these steps:

1. Change the DNS servers for the domain in question at your registrar to the ones for your account. You can find the ones for your account from your Customer area, My account, Manage - next to your hosting account.
2. Allow 24 hours for the DNS changes to propagate and take effect.
3. Configure your account to use your new domain. For more information on the available options you can check this link.