Perhaps this is obvious, but you need to enable ICS (Internet Connection Services) in order for this to work. Perhaps you have, but since no one has mentioned it, I thought I would. It's ICS that establishes the DHCP server for your client(s)!
Using ICS, the WAN NIC is transparently bridged to the local NIC where your clients reside. By default (iirc), ICS sets up a static IP for the local NIC of 192.168.0.1, then using DHCP, assigns additional IPs in the same 192.168.0.x network when those clients make their DHCP requests. As long as you enable ICS and connect the right cables to the right NICs, it all pretty much just works.
And btw, you rarely need crossover cables anymore. Unless you’re dealing w/ very old networking devices, almost all now support auto-mdix, which automatically configures the ethernet cabling correctly. So you can use standard ethernet cabling in almost all cases.