Or also you can extrude them in the reverse order from what I showed above, like extrude this piece:





Then Trim it with the other flat profile:





You can also use Boolean intersection directly on 2 planar curves to generate this type of result, it will extrude them out for you automatically.

But it's kind of easiest to do something like this by creating a solid from all flat curves and then cutting away from the solid.

- Michael