Is one supposed to first make the image black and white? Or make it color first? If for editing the tone curves, does one use the color tone curves or the black and white ones?
You start with a color image as input. The sensitivity of different b/w films to different colors is an important difference. If you start with a b/w images the simulation can’t look up how bright that particular color is supposed to be using this film. 
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I have no idea if this is technically the best way, but if I want b&w from a color image, the first thing I do is make it b&w, then I generally use the Lab curve adjustment.
I don’t like making the curve adjustment first because b&w conversion will greatly alter any other adjustment I’ve made.
If the intention is to use the RT film emulation, you should start with a color image. The lookup tables will map the colors to an appropriate B&W tone for the simulation you want.
Thanks @patdavid and @Jonas_Wagner - I think I’d tried it the other way before and now I know the right way.
@paperdigits - Interesting. What I usually do if I want a black and white image is to first make it a nice looking color image. Then turn in B/W. And then if I want to mess with the contrast any more, I use the Before and After tone curves under B/W. But I’ve been playing a bit with film simulation and wanted to know how that works. And, I hadn’t thought of this before, but I guess a B/W film simulation could also be used as a kind of HADCLUT-specific desaturation since one is able to alter the intensity of the film simulation. So there’d be a way to end up with muted colors.
I believe the film simulation is done via Hald clut.
Yup, that is true. It is made with hadclut
Actually, it’s called Hald CLUT.
Currently the Black-and-White tool lies after the Film Simulation tool in the toolchain.
Good to know. I need to make myself a flowchart one of these days to understand the flow since sometimes it can make a difference when trying to achieve complex looks.