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In Manual, the meter is always "locked". Whatever aperture, shutterspeed, and ISO the camera is set to, that's it. It's "locked" into that.
So, point your spot at the desired tone, center the needle by adjusting aperture or shutterspeed or ISO. That is now "locked" for that tone. Compose your shot, focus, shoot.
Remember that meters try to make what they measure into middle gray. Grass is about middle gray in tone. Caucasian flesh is about a stop lighter, maybe as much as a stop and a half. So if you meter on caucasian skin tones, open up the aperture a stop or a stop and a third, or slow the shutterspeed by that amount.