Ores

This tab lets you randomly distribute the ore deposits in your world. Your dirt layer must be dropped first, but otherwise you can do this before or after (or in between) biome generation.

You specify the percentages of each deposit, and the remainder (which is rock) will be automatically calculated. Note that the ore is also spread out in the water areas, not just the land mass, so keep in mind that if you only populate a tiny amount of a given ore, a portion of that amount may be unreachable.