How to Create Custom Victory Conditions for Scenarios?

I am making a rather large map set in late medieval Europe, and have realized that the victory conditions that I want are not possible without modding the game further.

Do you remember the custom victory conditions from the vanilla scenarios (such as conquer 4 civilizations or lose the game, as the mongols in their scenario)? How do I make my own custom victory conditions like in that scenario, and how do I make those victory conditions specific to certain civs? Such as in the scramble for Africa scenario, where N. African civs must gather gold, and European civs must discover natural wonders.

How do I do something like this?

Thanks in advance for your time and any answers you might have.

You'll need to change your map into a custom game, and then use Lua to write the various victory conditions - full details are given in this tutorial (it's long!)