The big difference between spot metering and evaluative, is that spot reads one small area, and ignores the rest of the scene. So the reading you get, is entirely based on that small area.
That's the easy part. Getting proper exposure doesn't come down to which metering mode you use. It has to do with how you interpret the meter's readings with regard to what your metering off of.
In case you didn't know (most don't), a camera's meter isn't designed to give you proper exposure. (I'll let that sink in for a minute)
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A camera's meter is designed to give you an exposure for middle grey. So it only gives you 'proper' exposure when you are metering off of something that is middle grey (or something that averages out to middle grey if you're metering on more than a small spot).
So if you spot meter on someone's face...you will only get 'proper' exposure, if that person's face/skin is close to middle grey (in tone, not color).
So what you need to do, to get proper exposure, it to either give the meter what it wants (meter off of a grey card, for example).
How to use a Grey Card ~ Mike Hodson Photography
Or you could meter off of something and make an educated guess as to how far off of middle grey that something is. For example, you could meter off of white snow, and guess that it's about two stops brighter than middle grey. So adjust your exposure to two stops above the meter's reading. You could meter off of green grass or foliage and in many cases, it's fairly close to middle grey. There are plenty of 'rule of thumb' type things but with a bit of understanding and experience, you can meter off of just about anything and make an educated guess that should get you pretty close.