How to activate a watch without previous owner?

Normally you can't. The watch is likely under Activation Lock which prevents lost and stolen watches from being used by others. The Lock can only be lifted using the original owner's Apple ID.

M1 Mac mini, macOS 13; Watch, series 7 GPS/cellular, watchOS 9.1; iPad Pro 2nd Gen, iPadOS 16.1; iPhone 12 Pro, iOS 16.1; TV 4th Gen, tvOS 16.1; HomePods, audioOS 16.1 and Airport Express w/ Airplay 2

if you mean activation locked then you can't really

the activation lock is not really on the ipad it's a theft protection measure on apples servers

it needs to be removed

About Activation Lock on your

Apple Watch - Apple Support

if you don't know your appleID Recover Your Apple ID - Apple

if that does not help you then you need the assistance of apple

Additional Support - Activation Lock (apple.com)

and or

Get Support (apple.com) ,

And or

 Genius Bar Reservation and Apple Support Options - Apple