There's no good way. Some carriers, like TruPhone, do have a stand-alone Apple Watch plan, but you have to set the watch up in "Family Setup," which means sacrificing the perks of a linked iPhone. It would be SO nice if one could add a secondary, Apple Watch-only eSIM to an iPhone with an existing plan (like Google Fi), or even just a data-only eSIM that the watch can use, but this unfortunately isn't possible.
Since Google Fi does support the Pixel Watch, they may eventually release support for the Apple Watch and other wearables, but it's unclear if this will ever happen.
In the interim, my temptation is to switch to something like Visible, which seems to have a relatively low total cost of phone + Watch plan (if you're OK with only domestic data) and perhaps switch back if Fi ever gets support.
Sigh.