Which model engine are you converting ? Mine was on a Lionel 2026 from 1951.
I did this awhile ago, so this is from "shaky" memory, but:
1. The wiring should be exactly the same as the existing. The only thing you are basically doing is swapping wick material and the heating element.
2. The black piece is the smoke pot cap.
3. After cleaning out the bottom of the old smoke pot, the white felt with the notch is the "liner" and goes into the bottom of the smoke pot. The notch should fit around the little pipe.
4. The white felt with the round hole is the gasket that seals the top of the black cap to the underside of the engine shell to help keep smoke going up and out the stack and fluid not leaking out and to keep excess heat from getting to the shell.
5. The wick slides over the heating (resistor) element. I don't remember it being critical which part of the smoke pot it sits in as long as it is fully enclosed when the black cap is on.
6. The spring is not part of the heater assembly. It is a replacement for the spring on the lever that pumps the smoke out if it is not pumping correctly.