how would someone who wanted to make a marble clock go about it?
Well, it involves a lot of different skills. Metalworking is important for creating the rails and bells, and there's a decent amount of architecture knowledge needed to come up with a design that suits the building it's made for. Most marble clocks were actually built by teams - an architect would be hired to come up with a design, working with a blacksmith to decide on which materials would be used, the details of the size and shape of the rails, etc. Carpenters, painters, sculptors and other artisans would also be brought in for different parts of the project (the decorated gates, the painted elements around the track, or even small sculptures fitted against the wall on some larger clocks). Oftentimes a mathematician would be consulted to help determine the length and steepness of the track, and to calculate the right weight for the marbles so that the clock could keep time. You've gotta keep in mind that the construction of a marble clock was a complex and expensive project, and could take months of planning before components could be fabricated.
If you wanted to make your own these days, you'd probably want to start with a CAD model of the room (or at least the wall) where the clock would be built, and then design a facade for the clock to fit on the wall in question. Once you've got your visual design planned out, you need to figure out the details of the track - steepness and length of different segments, number of loopbacks, etc. - as well as the right mass and diameter for the marbles (which is probably best done with a simulation these days). Actually constructing the components could maybe be done with a CNC machine and a lathe, but it wouldn't be easy, and of course you'd need the necessary skills to do the painting, carpentry and sculpting (if you're including sculptures).
All in all it would be a HUGE amount of work. It's why it makes me so sad that most of the surviving ones are in such disrepair these days. There's just such a small likelihood of anyone ever building a proper modern one.
