It always depends on what you are trying to achieve. Slower exposure times; use an ND. Reduce or kill reflections, use a CPL. Do both, use both.

That said, you will want to put the CPL on first, adjust it for effect, then add the ND filter and adjust exposure. Of course, you want to ensure the CPL filter is in the original position after adding the ND filter. I use a silver Sharpie to place some marks on the outer ring of the CPL. It's easy to see where those marks are after adjustment, and return the filter to that position after adding the ND filter.

Note, I'm not saying to do the marking for this exposure; just having some marks for reference makes it easy to return to approximately the same position.

As an alternative, you could use tape to lock the CPL in place; piece of gaffers tape would do the trick. That's just another thing to fiddle with, which is why I just marked up the filter.

The marks can be easily removed with alcohol or goo-gone, if you decide to sell your filter in the future.

If they are both very high quality, multi-coated filters, should work fine. In fact, it's a common technique to do so, when the ND filter alone isn't quite strong enough... Or when wanting both to reduce light (ND) and cut some reflections or glare (C-Pol) in a scene. ND filter alone can't do that. OTOH, there are times when you don't want to reduce reflections because they are an important part of the image', so will want to avoid a C-Pol.

Test on your lens to see if you have problems with vignetting when the filters are stacked.

I also wouldn't use a "Variable" ND in combination with a C-Pol. Variable ND are essentially double stacked C-Pol and are already prone to uneven effects... adding a C-Pol to that would probably make for even worse unevenness issues.

Contrary to an earlier response, there's no problem adjusting a C-Pol when it's stacked with another filter.