ADC eliminating noise for improved accuracy
24 bits is hard. luckily most 24 bit ADC's have an ENOB (effective number of bits) of around 19, which means your measurements "only" need to be good to 2ppm of the input voltage range, 

Supply noise: This is where you pull up the PSRR (power supply ripple rejection) of your ADC and work out how much of an error source a noisy supply will cause, in these cases I tend to break these charts up in to frequency bins so I can add up all the noise sources for different bands to know exactly how I need to fight it, independent noise sources add as the sum of square roots, 

You can also pull up the expected noise output graph of your power supply and see how much you need to fight it down

Then there is thermal noise to add in to the mix, as the ADC will have its own internal noise in the measurement

Next up external noise sources, for this level of resolution, you want differential measurements, this makes it easier to remove most of the impact of common mode noise, and when differential the ground in your PCB is acting more like a shield than a signal wire, on top of this you will want some capacitance between the differential lines to soak up the differential mode noise. 

For any capacitors in this circuit, I would also recommend NPO or COG, you do not need the capacitor being part of the noise source.

Shielding is ideally to your circuits ground, you want that enclosure your chip is in to always remain the same voltage as it, otherwise it will start coupling in to sensitive nodes.