Hello Nagulaz,
I don't know of a tutorial, but I just dealt with this problem. With help from others in this forum, the main solution can be found here (http://forums.dlink.com/index.php?topic=8786.0). Simply put, you can connect the 1st router's LAN port to the 2nd router's LAN port if you want both routers to be all one network, or connect to the 2nd router's WAN (Internet) port if you want to create a second network with greater security. That would give you a second firewall to hide your basement machines behind, so you can save the upstairs computer for risky stuff.
But using a DIR-615, the wireless connectivity could be your biggest problem! See this forum (http://forums.dlink.com/index.php?topic=8415.0) for the solution to a BUG in the DIR-615.
And remember this simple rule: when first trying to establish wireless connectivity, set the security to DISABLED in the wireless router first. Try to establish a connection to your wireless machine (laptop?). If you can connect, then go turn on security and try a simple network key in each machine, choosing what type of security you want (WPA, WPA2, etc.). You'll have to go change the settings in your laptop at the same time. If it works again, then finally go to www.Wireshark.org and follow directions to get a really secure private security key. Plan on doing a copy/paste to put the key into each machine.
Good luck!