It may seem straight foreward and it can be :) - but there are several different ways to connect the OpenWRT router Digipeater / iGate. I will try to provide some hints as to how it can be done here. It is not meant to be a bible, just a guide so everybody have a chance of a quick start.
I will try to explain som basic scenarios:

Well - is it too trivial? fine then you probably dont need this info.
The default router IP address you shall use when configuring the router is http://192.168.1.1 the username: admin and password: 'nothing' , first thing you shall do is to set your own password. 

This probably the easiest option, and lots of people have done this before. Just connect the internet to the WAN connector on the router, and any extra equipment to the four LAN ports.
Since this only is about connecting the router you are done now, but you should configure the Wifi to get the most out of the router.  

There are two ways to connect to your existing network using cable (there are more ways, but even more complex), you can use the LAN ports or the WAN port. Both options are valid but also has their own issues to be aware of!

1. LAN port connect, you MUST disable the DHCP server on the Digi / iGate router, and at the same time set a fixed ip, select one outside the DHCP range of your main router. (write it down, it's so easy to forget)
2. WAN port connect, the easy way - just connect and your are ready. The LAN ports are not very usable likewise the Wifi since they are not part of the existing network. Futhermore to get to the internet there is an extra router in the path. One thing to remember though is to enable WWW and SSH through the firewall or else you must connect and do your configuration work from one of the LAN ports.