How to report a stolen Apple Watch?

Apple doesn't get involved in lost or stolen goods, so your only real option is to contact the local authorities where the watch was stolen and if you have it set up for cellular, contact your carrier and ask them to blacklist the watch.

Otherwise, follow the suggestions in this link --> If your device is lost or stolen - Apple Support

Report it to law enforcement. See information in If your device is lost or stolen - Apple Support

For an iPhone, also see: iPhone Theft and Loss Claims - Official Apple Support

and

Use any internet browser and log into www.icloud.com/find

Also see the following Apple articles for information about lost or stolen iPhones or iPads: Find your lost iPhone or iPad with Find My - Apple Support and

Find your lost Apple device or AirTag with Find My - Apple Support