How do I create a forest root domain?

In the console tree, right-click the domain node for the forest root domain, and then click Properties. On the Trust tab, click New Trust, and then click Next. On the Trust Name page, type the DNS name (or NetBIOS name) of another forest, and then click Next.
For More Information Please Refer:
https://social.technet.microsoft.com/Forums/en-US/f5a4483a-d1ba-48ca-b2f6-07efd640d249/guide-on-creating-trust-between-two-domains
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