I’d like to get mint mobile. How do I get Apple Watch cellular

Sure. Choose another carrier who supports Apple Watch. The link I provided tells you who does. You're not going to get cellular on an Apple Watch with Mint Mobile.

Whether Mint Mobile supports iPhone, you'll have to ask, but according to Apple (see the link below), they do NOT support Apple Watch. So having Mint Mobile on your Phone, means you can't have cellular from them on an Apple Watch:

Apple Watch - Carriers - Apple