Modded PS2 or Softmod: You will need a way to run homebrew software on your PS2 (such as FreeMCBoot).
USB Drive: Insert a USB drive into your PS2.
BIOS Dumper: Run a BIOS dumping application (a common one is simply called "BIOS Dumper").
Transfer: The program will copy the BIOS file from the console's chip to your USB drive. You then move this file to the bios folder in your PCSX2 directory on your PC.