How can I set up Family Sharing for Apple Watch?

Short answer: you don't.

Long answer:

Family Sharing is not setup for specific devices, it's set up for Apple Accounts, and devices attached to those accounts can then access the subscriptions and purchases through Family Sharing.

What exactly are you looking to have the Apple Watch access from Family Sharing?

Or do you perhaps mean, Family Setup, where you set up an Apple Watch for a family member that does not own an iPhone. click here ➜ Set up Apple Watch for a family member - Apple Support

If you mean something else, then please clarify.

Hi mfb83,

Re: Family Sharing, Set Up Apple Watch cellular issue

Wondering if Apple Support's online article (link below) can help guide you through the steps:

Set up Apple Watch for a family member - Apple Support

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Re: I try to set up her Cell service and it keeps directing to log into my network and not an option for her service.

Do you want your daughter to be able to use your wi-fi ? (ie: as well as her cellular service for calls etc.)

See step 7 in the above article for details.

( You would contact the provider to set up your daughter's service. )

Hope this helps :-)