I assume you're talking about using a wireless router as an access point. First, you need to run a wire from on of the LAN ports the new wireless router to one of the LAN ports of the new wireless router. (The back of your router has one Internet/WAN port (often yellow or blue) and several LAN ports.) The simplest way is to use a Cat5 or Cat6 patch cable, which you can buy in any length from 1 to 300 feet. (It's possible you'll need a crossover cable, but I haven't needed one of those in years.) If you're thinking of putting them in different parts of the house, and you can't run a cable between them, then you probably can't do this. (There are devices that are capable of creating a wireless bridge, but you'd need both devices to share that capability, and it's not typical on home devices.) You could try getting a pair of devices that use your house's electrical wiring (like these http://us.dlink.com/product-category/home-solutions/connect/powerline/
), but that's not always successful (you need both outlets on the same circuit, for one thing).
Second, you need to change the configuration on the old router/new access point. That box is doing a lot of things that you don't need it to do any more, but I don't bother disabling them, except for one thing: DHCP. Wireless routers are generally configured to hand out IP (network) addresses to other devices on the network (your computers, DVRs, game consoles, etc.). If you have two devices (your new wireless router and the access point you're creating) handing out network addresses, it can get messy. So you need to log into the old router and disable DHCP (which some manufacturers call "assign network addressing" or "IP address distribution" or something like that).
One other step you may need to take is to change the IP address of the new access point. Still logged into the router, find where the LAN (local) IP address is set, and change it if you can. There are 2 reasons this can be important.
1) If your access point and your new wireless router both have the same IP address, you'll get lots of problems.
2) If your access point and your new wireless router are not on the same subnet, you'll only be able to login to the access point through a wired connection (and even then you may have to change your computer's IP address to match the access point's subnet.)
You can change the access point's address to anything you want, but you need to make sure of 2 things:
1) No other device has that IP address.The easiest way to ensure this is to make the last number in the address 253. That address almost never gets used.
2) It's on the same subnet as your new wireless router. I won't get into subnetting; what you need to know for your home network is that devices on the network can only communicate with other devices whose address starts the same way. For all the home routers I've seen, it's either 192.168.0 or 192.168.1. So you need to log into your new router and see if it has the address 192.168.0.1 or 192.168.1.1 (or you could open a command window on your computer and give it an "ipconfig" command, which would show you your computer's IP address, which will have the same 3 numbers). Then login to the access point and give it the IP address 192.168.0.253 or 192.168.1.253.
In a nutshell, make sure the first 3 numbers match all the other devices on the network, and that the last number does not match any device on the network.