Hi Andi,
Think of it as a road interchange, the car/packet needs to know which road/route to take to get to its
destination, and the destination needs to know what road/route to take to send the car/packet back to its
starting point.
Draw a diagram with your computer/modem/router etc on it, with a line connecting the route you wish to
take. Each device needs to know where the next hop is, don't worry about where the box/pc/next hop is
after the immediate next hop is.
The computer with two NIC's installed will need both interfaces setup, and a routing object setup so the computer knows
that any packets not addressed to itself should be instead forwarded to the next hop.
An example x:\mptn\bin\setup.cmd looks like
route -fh
arp -f
ifconfig lo 127.0.0.1
ifconfig lan0 192.168.1.1 netmask 255.255.255.0 metric 0 mtu 1500
ifconfig lan1 10.0.0.2 netmask 255.255.255.0 metric 0 mtu 1500
route add default 10.0.0.1 netmask 255.255.255.255 -hopcount 1
inetcfg -s all
ipgate on
If you enter 'route' at the command line by itself it will give you plenty of ways to spend hours being totally non productive but learning much

In the above setup.cmd file,
"route add default 10.0.0.1 netmask 255.255.255.255 -hopcount 1"
tells that computer that all other traffic should be sent to the address 10.0.01 by default.
10.0.0.1 is the next 'hop', or next computer/router/modem in the chain that data is sent to from this computer which has 10.0.0.2 as its interface to the 10.0.0.x subnet.
If you do not use x:\mptn\etc\inetcfg.ini then remove the line "inetcfg -s all"