It depends on your hardware. Some hardware has a separate Ethernet port on the CPU that goes directly to the WAN port on the back of the router. In that case you would go to the network configuration, physical settings, and move the WAN ethernet port from the wan network to the lan network.
In other cases, especially gigabit routers, all the ports on the back go through a hardware switch. There you would go to the Network-Switch page and turn off the WAN port in the WAN VLAN, and turn it on (untagged) in the LAN VLAN with the other LAN ports.
DDwrt tries to simplify configuration by giving the user preset "modes" for common use cases. OpenWrt is more toward total flexibility. You can set anything to do anything, even if that is going to totally break it. You can add or edit a file anywhere in the filesystem rather than having only a "nvram" database.
If you're using command line, the most direct approach is to edit the config files. Search the OpenWrt wiki and find the UCI pages. These explain the options possible in each file.