Conceit comes from the word concept, so think of it as an idea. A metaphorical conceit is described as an extended metaphor with complex logic. "Complex" just means that the metaphor continues to be used throughout the poem/passage in different ways. For example, if you compare death to sleep, to make it a conceit, you would have to do more than just say something like "to die is to sleep, never to awake." Also, the metaphor of death as sleep has been "done to death." Conceits are often praised not just for their complexity but for their strangeness. The more odd the comparison, the more striking the conceit.
In John Donne's "The Flea," the speaker compares sex (or marriage) with the flea because the flea has bitten each of them and their blood is now mixed. The speaker uses this as an argument to get the woman to make love to him. Since their blood is mixed already, it's as if they're united, perhaps in marriage.
O stay, three lives in one flea spare,
Where we almost, yea, more than married are. (10-11)
What makes this a conceit is how dissimilar the elements being compared are and the fact that, being so dissimilar, the comparison must be explained because it isn't an obvious analogy such as "death" and "sleep." The further explanation that's required is how the metaphor is extended. Since it isn't an obvious metaphor, it must be extended.
So, try thinking of a strange comparison to death. A way to make this more interesting is to personify death. Maybe Death is a chemist who turns a solid into vapor (a body into a soul). And then extend the metaphor from there. Death does experiments. Sometimes the elements live a long time; sometimes they don't. Each experiment eventually ends in a transition from one state of matter to another. Or, death could be an event or just some thing. Maybe death is a certain kind of hydrogen atom that is emitted from a person when they die. Being the lightest element in the universe, this hydrogen atom (for whatever reason you make up) can form no material bonds, and must float up.