Three or so different ways, depending on camera, film and subject:

BW / LF: spotmeter shadows, check highlights and midtones to see what will happen if shadows are placed on zone III. Adjust exposure until feasible with home-mixed chemicals. Then forget my planned development and develop by inspection anyway. Or stand development in FX-2 if I have truly no idea of what it is.

Colour: Aim incident meter backwards over my shoulder, set speed and aperture accordingly (outdoors).

Old Folders: "Debatometer"! Look at the sky (or other light source), guess how strong the light is compared to "sunny 11" (empirically determined for 60 north latitude), open up for clouds, maybe close down for old uncoated lenses (less contrast due to flare), wiggle the dials a bit, look at the sky again, guess what the longest time I can hold the camera still is, set the aperture I want, look around for nice solid support. This sounds nowhere near as haphazard as it really is, but I get a surprising number of well-exposed negatives this way! I justify it as being "historically correct". Or should that be "hysterically incorrect"?