It´s quite a general question, so what exactly do you need to know.
SPI (hardware, timing) is well standardized, so it´s in most cases independent what SPI partners you use. For sure you need to send the correct bytes as described in the datasheets.
Wiznet has published example software, e.g. for AVR and STM32. You can use it as a template for your PIC18 design, just replacing the low level SPI routines. For the hardware setup, you just need to know the SPI mode and supported clock speed.
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I just saw that Microchip has a W5200 PICtail, to be used among others with PIC18 development boards. So there's probably readymade software from Microchip.