You've got about 4-5 stops to work with with slide film.

If you've got a relatively low to normal contrast scene, you can meter from a grey card or take an incident reading or from a midrange tone in the scene, color perception being a relatively midrange phenomenon to begin with.

If you've got a wider range, you need to make a judgement about what is important, and what you are willing to lose. If you have a relatively brightly toned subject and want to keep detail in the highlights, you might meter the highlight, for instance, and place it about 1.5 stops above middle grey. If you have a very dark subject, and that is what is important, you might place it 1.5 to 2 stops below middle grey. If you are photographing penguins, wait for an overcast day.

And then, there is a range of exposures that might be "right", but you can get a very different tonal effect by bracketing 1/3 stop up or down. A slight reduction in exposure will give you a little more saturation. You also might find that an image that looks good about 1/3 stop under on the light table doesn't scan as well on a desktop scanner as a slide that is a little lighter.