Hello @Splatspeed
If you don't want to use NXP library to develop, then two suggestions for reference:
1. Between Master and PN518 is the SPI interface, there is a standard SPI communication protocol, no special application description here. PN5180 datasheet is open source. For PN5180 Antenna design, you can refer AN11740.
2. Between PN518 and Tag, this application varies according to the tag. If it is NXP's tag, you can download the relevant development documents from the official website.
Best Regards,
Thanks,
kelly