Low enough that you can cut the grass right to the edge but not too low.

The soil level should be more or less the same as the lawn, but pushed away from the edge of the lawn, either with a spade or half moon edger, so there's a slight drop at the edge of the lawn where the two meet. Sometimes you see a border that's a good bit higher than the lawn, which can be hard work because when it rains, it tends to wash onto the lawn. Even if yours looks a bit too high after digging and adding composted materials, it will settle after a while, and the organic material will degrade further, and the level become a bit lower.

When you have a little ditch between the bed and the lawn it discourages lawn from getting into border and it makes edging the lawn a lot easier.