Hi. how can i install my apple watch to someone else iphone (i gave to this person my watch as a present)

Before you sell your Watch... -

https://support.apple.com/en-us/HT208855

ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2.8; iPad Air 2, ipadOS 13.6.1; iPhone 8+, iOS 13.6.1; TV 4th Gen, tvOS 13.4.8;  HomePods, audioOS 13.4.8 and Airport Express w/ Airplay 2