If you wanna level both, that’s fine too. I know of three good ways, all require a flat spot on the stock or action. Level the stock/action, then:
1) Hang a plumb line at distance, align the vertical reticle to the line (requires some space, ideally 30-50 yds minimum)
2) Toss a clear water jug on the ground at distance, about half full, then align the horizontal reticle to the water line (requires some space, etc)
3) Mark the diopter setting on your scope, then hang a thin string a couple inches off a light-colored wall (I use dental floss) with a weight on it. Set the butt of the rifle near the wall, then shine a bright flashlight into the objective lens. Adjust the rifle location and diopter setting to make the reticle shadow clear on the wall, then align it to the string shadow.
I usually use option 3, even though it’s more complicated, because I can do it anytime in the house with no distance required (I live in the city).