Hello eam!
1. You have to find your fellow players on your own, there is no central game server ...
Ask your friends or find someone here in the forum ...
2. Install RnD and the levelsets you want to play on every PC.
3. Create a shortcut to rocksndiamonds.exe and change the target property from something like
C:\Program Files\Rocks'n'Diamonds\rocksndiamonds.exe
to
"C:\Program Files\Rocks'n'Diamonds\rocksndiamonds.exe
" -n
The path depends on where you installed RnD, important are the quotes and the -n (network mode) command line argument.
4. Start RnD with this shortcut, this game will be the host, you need to know the ip address of this PC.
5. Your fellow player creates and runs a shortcut like this:
"C:\Program Files\Rocks'n'Diamonds\rocksndiamonds.exe" -n x.x.x.x
where x.x.x.x is the IP address of the host.
6. Then just select the same levelsets and start a game!
If someone has a firewall or a router, you could get problems with the connection and the ip address; personally, I use
Hamachi (see this thread
viewtopic.php?t=732).