First, you will need to go to the Domain Search Page

Once you're there, write the domain name you'd like to purchase, and select the extension required, then click on search
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You will then see whether the domain is available or not, and have the ability to register other alternatives as well we suggest to you.
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When you're ready to proceed, just click on Continue button at the bottom and it'll take you to the order form where you can complete your purchase.