How?
e.
I presume you are asking about the 24mm lens with full dedication for the Contax 645? (which is fully compatible with Leica S too via the Leica S adapter)
There are 3 24mm lenses one can convert for use with the Contax 645... (that will fit physically)
1. The Samyang 24mm f3.5
2. The Pentax 645 25mm f4 and
3. The Nikkor 24mm f3.5 PC-E ...
The Samyang offers the wider image circle among them all (it will cover even a 54x40.5 image circle easily), it's easy to convert but its IQ is not acceptable until its closed to f8 and then its not as sharp as the others... Additionally it will only work stopped down.
The Pentax is the easiest to convert as there is no conversion involved with the lens and it requires for one to make an adapter of 6.7mm width, which will have an aperture ring on it and then, link mechanically the aperture ring to the lever that controls the aperture on the lens mount... The lens is sharp, but again it can only meter stopped down and is very expensive.
The Nikkor 24mm f3.5 is as easy to (physically) convert as the Samyang, it requires to remove the rear part of the lens (very easy to do) that has the mechanisms for shift and tilt and make a "tube" instead that extends from the aperture ring to the Contax mount (same with the Samyang) which needs to be 17.5mm shorter than the ordinal lens length. Fortunately, (on both the Nikkor and and the Samyang - unfortunately the Canon 24mm T-S cannot be converted physically) the rear element when the lens is focused to infinity (its longest extension) is recessed enough inside the lens body and thus it leaves another 4.4mm before it messes with the Contax mirror... Now that ends the physical part... (which is by far the easier).
Then, one has to rehouse all the electronics of the original lens back inside the new lens rear "tube" but
add a small PC board that will convert the lens interface from Nikon E to Contax 645... After this you are done! The lens will work just like if it was a genuine Contax 645 lens, with full interface communication and focus confirmation but no AF of course (only because there is no AF motor on the original lens, if there was, this would also work), but with a 24mm lens who cares for AF anyway?
The Nikkor has smaller image circle than the Samyang as it projects heavy vignetting on a 54x40.5 sensor, but it covers the 49x37mm sensor very well (about 2.5 stops of (correctable) vignetting at the last mm of the furthest corners), it is the sharpest out of all (sharper than the Canon 24mm TS too), it is even extremely sharp at f3.5 but to the edges which are still usable wide open, it is very sharp across the frame from f 5.6 up to f16 (with a 6.8 μm pixel size - f11 with smaller), it is the most flare resistant out of all, distortion is very small and only has some chromatic aberrations which (as well as the small distortion) are easily and automatically correctable when processing (by one choosing the automatic lens profile in LR).
If somebody wants to have his lens converted for Contax (or Leica) don't hesitate to contact me... You only have to buy a S/H 24mm f3.5 PCE Nikkor lens and I'll do the rest for you.