I don't have one as I've been doing mostly openwrt but they are very likely wells documented in the dd-wrt manual.

Look for how to flash than onboard switch configuration (in OpenWRT this is done by ending th switch switching of the network config file ) (be careful of the hardware flaw we found where a port is tagged and untagged at same time)

At that point it's operating as a switch and your set for that and you can extend the operating system to handle routing if needed as well.

Here is a guide I've given out as a starter before:

http://www.dd-wrt.com/wiki/index.php/Switched_Ports