Is there a guide I can follow to set up the span port through the GUI? I currently have WAN and LAN as member interface. I want to set it up such that a computer in the LAN can capture using tcpdump all incoming and outgoing traffic of the whole network.
Which member interface to I choose? And do I have to create another interface since it's stated the span interface cannot be part of the member interface? I'm using VM to test it out if it matters.
Yes. If you really want to try to use pfSense for this, you have to add another interface to the bridge and use that as a SPAN port.
Which do I select as the member interface if I want to capture everything. Wan or Lan?
The SPAN port will reflect all traffic on the bridge so you would have to add anther port, set that as SPAN and then connect the tcpdump client to that.