PPTP setup on DD-WRT has a high rate of failure, as such this guide is provided "as is" - due to the large number of routers that are out there - and the impossibility of covering them all, your use of VPNinja on your PPTP should be considered an adventure for yourself.
PPTP cannot be used concurrently with PPPoE or Static IP.
The WAN port on the DD-WRT router must be directly connected to either a) the ethernet port on your modem or b) a LAN port on an existing router for this setup to work properly.
Once PPTP is configured on the DD-WRT router, all devices connected to that router will be routed through VPN.
Make sure you don't use the same IP as your main router. Your non dd-wrt router will probably be on 192.168.1.1, in this example we used 192.168.2.1.