Hello Sniglit,
Thanks for posting in Apple Support Communities. We're happy to provide information about setting up cellular data on your Apple Watch.
Your iPhone and Apple Watch must use the same carrier and you must be within your carrier's network when you set up cellular on your watch. The only exception is if the watch is set up for a family member who doesn't have an iPhone.
An Apple Watch with cellular does not require a physical SIM card. The support article that you posted from walks you through the steps to add your plan: Set up cellular on Apple Watch - Apple Support
Cheers!