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Literally just use any light meter.
The first camera I ever owned was a Zenit-E. A soviet SLR from 1965. It has a built-in light meter, but not the way you'd expect from other, non-autistic SLRs. It had a selenium cell above the lens, so absolutely no point-metering. The more light that hit the cell, the more voltage the cell produced. This voltage moved a single needle on the top side of the camera, which you had to align this (pic relates) exposure dial with, then align the inner ring with your ISO, then read off of it. The inner dial indicates your aperture selection, and the outer your shutter speeds. If one is vertically aligned with another, those settings will result in a perfectly exposed photo.
That's it. This is as simple as it gets. I never shot a manual before this, I had no idea what I was doing. Despite this, ny entire first roll came out perfectly exposed (and I'm an idiot).
Just use any light meter.