Once again, if you're asking these questions, you're probably not ready to be designing your own Ethernet PCB. Truly don't mean to offend, but engineering complexity of these devices is a bit higher than an Arduino clone for e.g. Rolling your own is likely to be a frustrating and expensive endeavor until you have the experience to understand the reference schematic, and how to lay out your board accordingly.
As I posted in this similar thread
, if you don't understand these questions, you should probably hold off on the DIY design (for now) and use a pre-built module:
As a practical skill assessment, are you using jacks with integrated magnetics, or are you using an external isolation solution? Is it compatible with auto-MDIX? Will you be using PoE, and if so, does your isolation mechanism provide the appropriate sense resistors?
If you're dead-set on doing this (hey, everyone's gotta learn somehow), make sure you post your design for review before sending away for PCBs. Also, expect and allow for some duds at first. You'll get there.