Everything is over here:

https://git.kernel.org/pub/scm/linux/kernel/git/maz/cs-hw.git/tree/README.txt

* Building your own:

This is the preferred option, really. I've used JLCPCB for all the
revisions above, both good and bad. If you're not good at soldering
very small stuff, get them to do the heavy lifting. The process is
very straightforward, but you will have to produce at least 5
boards. Find some fellow hackers and share the costs!

The 'production' directory contain the Gerber files in a single ZIP,
BOM and positions in CSV format (produced by the JLCPCB fabrication
toolkit plugin). You only need to upload those to the JLCPCB website,
check the orientation of the components, and let it rip. I've used the
basic FR-4 with HASL finish, Economic PBCA type, and the result is OK.

Things to be aware of:

- The assembly side is at the *bottom* of the board. It is only the
  Pico that goes on the top side. Make sure you pick the bottom side
  in the web interface.

- Warning about JP[1-8] being absent from the position file can be
  safely ignored. The web interface gets confused about having pads
  without anything soldered to them.

- Components sometimes are out of stock. While you can often quickly
  find a direct replacement in the LCSC library, be very careful about
  the footprint, specially with connectors. You may have to amend the
  PCB design and regenerate the production files.

- The orientation bit is absolutely crucial. While I do my best to
  reconcile what KiCad and JLCPCB respectively think of the
  orientation of components by adding rotation offsets to the
  metadata, you absolutely need to check this carefully before
  starting the build. Look for the orientation markers on the PCB and
  use the web editor to align the component positional markers with
  them. Do not expect JLCPCB to catch these mistakes for you.

Of course, JLCPCB isn't the only game in town, and their QC is dodgy
at best. If you know of a decent PCB+assembly shop and can help with
making the PCB easy to get produced, let me know. I'm happy to add
metadata to the schematic to drive a fabrication plugin/framework if
there is one.

Ideally, I'd like to have a collection of setups that allow people to
build boards locally instead of having shipped around the world...

You can also use one of these PCB shops to only produce the PCB and
populate the board yourself. If you are in this category, I assume you
know what you are doing and you need no further advice from me!

* Final assembly:

Once you have managed to get your hands on an populated CS board, you
must solder a Pico to it. Make sure that:

- the two boards are back to back (all components are on the outside
  of the board sandwich).

- the two micro-USB connectors on the same end of the assembly -- if
  you have the USB-C and Pico micro-USB close to each other, you're
  doing it wrong.

- the two boards are far apart enough that the two micro-USB
  connectors can be plugged without interfering with one another.
  I've used male turned pin strips as the connector, and they are
  great.  Normal header pins are also fine if you're not bothered with
  one side sticking out more than it normally should. You may need to
  add extra spacers to keep the boards apart.

- you use a bog standard RPi Pico. Not a Pico W, not a one of the many
  variants with a creative pinout... It may work, it may not. Be
  cheap, don't use anything fancy. The Pico W is known to have a
  different GPIO assignment, which interferes with the current
  SW. Nothing terrible, but enough to spend some time debugging it.