How do I install my SIM card In my Apple Watch

You don't.

The Apple Watch cellular uses an esim. You activate it on your cellular plan using the Watch App on your phone if you're using a supported cellular carrier.

https://support.apple.com/en-us/HT207578

The Apple Watch shares your iPhone's phone number. To do this the iPhone cellular provider has to be capable and willing to have one 1 number be used by multiple devices. For this service, supporting cellular providers charge an additional monthly fee to your iPhone bill. In the U.S. it is $10 per month.

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular