It is people on instructables giving bad advice again. The first one won't ever work as you're wiring all 3 sockets in parallel. One of the pairs will have the transmitter from one of the hosts and the monitoring machine connected together which it won't like. Assuming it didn't mind that, it wouldn't be able to see what was going on with that pair anyway since the monitoring host doesn't have a reciever on there.

The second one is how you do it but you need two NICs. One to monitor incoming and one to monitor outgoing.

The other option would be to find a 10/100 hub (can you even buy these things still?) or see if your switch has SPAN support. If it is a basic unmanaged home switch then it won't.