Search for Google Pay within Play Store on your handset.
If you've not got Google Pay on you phone then it will prompt to install to both handset and watch, if you've already got Pay on the handset then the install option will show for the Watch (there's a drop down at the side of the 'install' tab which should list tick boxes for 'this device' and 'Watch 4' ).
You can also use the Play Store app on the Watch and search for Google Pay on that.
