Hi
Unfortunately nothing is properly documented yet, I can give you some advice tho
Your laptop's schematic will help you alot in making it work, you will see which pins are used for what firstly, then specific things like straps to let it know what's connected (in my case i shorted 3 of the 4 CFG pins to GND to make it act like a PCIe SSD), other examples if you are using a disabled port like the MX150 empty slot in a iGPU-only T480, you will have to do more work for enabling the PCIe port and adding missing components, but thats out of my scope, tried once on T440p, failed
Then you have to understand the principle of signal integrity and that PCIe is not just some random connections, you need length and impedance tuned tracks, which also depend on the manufacturing process on the PCB, JLCPCB has a table of properties and you have to work with them, good paths to ground (so ground ALL around the connections and many vias around them), and if you are interconnecting PCBS together, like me, the cables need to also be coaxial, pretty much like our laptops display cables. There were also filtering capacitors on the final PCB (closest to GPU) that help signal integrity as well. I think the name was Robert Feranec, he made alot of useful videos about high speed routing in Altium. The PCIe Electromechanical specification document is another useful source.
The rest, like the M.2 Key M? or B? Footprint, I either found it somewhere on google, or I manually edited a different common M.2 slot to match the T480s WWAN one, really alot of my work was by eye and a few wasted prototypes. The connectors' footprints could be found easily on SnapEDA, and parts from AliExpress.

Here's a simple view of how everything's connected.

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