Diomedes build guide:
THE TRRS CABLE IS NOT HOT-SWAPPABLE, ONLY UNPLUG THE TRRS CABLE AFTER THE USBC CABLE IS DISCONNECTED, DAMAGE TO THE CONTROLLERS MAY OCCUR.
Firmware is available on the Downloads page.
1 - Flash your KB2040, download the Diomedes KB2040 Vial firmware, plug the KB2040 into your computer, quickly double-tap the reset button. You should get "drive" popup, simply put the UF2 firmware file into the drive. Repeat this process for the other KB2040, both controllers get flashed with the same firmware. Congratulations, your controllers are flashed!
2 - You can verify that your KB2040's has flashed correctly by checking Vial.
ALL COMPONENTS BESIDES TRRS JACKS GO ON THE TOP OF THE BOARD
3 - Begin bending and soldering all diodes on the board into the spots marked D1-47. Black stripe of diode goes towards the black stripe marked on the PCB. All black stripes go towards the bottom of the PCB's. It is imperative that you do not get any backwards. 
This part is fairly difficult on the KB2040, Don't do them all at once, do one line at a time, trim the legs as flush as possible, then continue to the next line. The diode legs are pretty close together and it is easy to bridge diodes, be careful here and use a nice pair of flush cutters to trim the legs when you are done. This is absolutely the hardest part of this assembly. Inspect all of your joints, ensure you have no diodes that are touching each other.
4 - Now, we are going to solder in your KB2040's. ORIENTATION OF THIS MATTERS VERY MUCH. The USBC port obviously goes to the edge of the boards, the components of the dev boards face TOWARDS YOU, away from the PCB, flat part of the KB2040 goes against the PCB. Both controllers are placed on the top of the PCB's. The bottom of the PCB's have the Diomedes text as well as the D(X) designations for the diodes. Take one of the pin headers that come with your KB2040's, put the long side through the bottom of the pcb, the black plastic should be on the bottom. set your KB2040 on the pin headers, lined up with all of the pads. pick one pad on the side without the pin header and solder solder the kb2040 to the PCB. Remove the pin header, ensure your placement looks good. If it looks good, continue soldering the rest of the pads to the PCB.
5 - Now solder the TRRS jacks on, these are mounted to the bottom of the board, indicated by the silkscreen on the PCB. Make sure they are flush against the PCB before you solder them in.
6 - At this point, I STRONGLY advise you test your board by shorting the switch holes together with a suitable tool and using Vial's matrix tester tool. It would be a real bummer if you soldered all your switches on, just to find out you soldered a diode on backwards. Again, plug in your TRRS cable to both halves, then plug in your usbc cable to the left half PCB.
7 - Now begin inserting your switches into the PCB. This part is pretty self explanatory. Insert all the switches and solder both pins from each switch onto the PCB. Make sure you have no bent pins, and be careful not to scratch the PCB, if you opted for an encoder, install that instead of a switch on the right half. Do the encoder last, it can be difficult to get all the switches laying flat against the PCB.
8 - The board is fully functional at this point, so we will move onto the bottom plate. This is simple, install the standoffs to the bottom of the PCB using the provided screws. Once all standoffs are installed, gently place the acrylic bottom onto the standoffs and screw it down. Do the same for the acrylic window. Please note, the acrylic windows only line up correctly one way.
9- Install your keycaps, its ready to go!
If you see anything in the guide that needs attention, let me know. If you need any help whatsoever, don't hesitate to reach out to me on discord, thank you so much for your support.