First thing is, since this is not a normal home setup, you will need to be familiar with the way IP networks are configured. Because you won't be able to just plug things together and expect them to work, straight out of the box. If you need to become familiar with IP subnets and routing concepts first, the I would suggest to read this RFC tutorial before going much further:

http://www.rfc-editor.org/rfc/rfc1180.txt

Also, what do you mean by "WiFi modules"? If you mean access point, like a box that transmits and receives packets over a wireless link, that box usually incorporates a router. The ones I see in stores are all called "WiFi router," and this is why. The router is built in. You should not need another router.

I attached a picture that shows what I think you will need to do, if I understand what you mean by "WiFi module."

You should see that each WiFi router has a specific Ethernet port, normally used to tie into your home modem. That port probably has a connector of a different color from the others. Then you may have a few other Ethernet ports, which are ones used to connect to any hardwired PCs you have at home.

The wireless network will belong to the same IP subnet as those local Ethernet ports, and it will be a different IP subnet from the one at the unique port meant to connect to the modem.

Assuming I have understood "WiFi modules," then to connect several of these together, in a network with multiple WiFi routers, the general idea is to create a "backbone Ethernet," to which you connect the unique modem port of every WiFi router. Then configure the IP addresses of each WiFi router, using a scheme such as the example in the attached file. (Those are just examples of addresses, but whatever IP subnets you choose will have to follow this scheme.)

You would enable the DHCP module in the wireless LAN of each WiFi router, so that PCs connecting to the WLAN will obtain a valid IP address automatically. The ports in the backbone LAN have to be statically configured.

And you will want to enable RIP (routing information protocol) at each of the WiFi routers, so each router knows how to reach the various wireless LANs at the other routers.

In the attached file, I use the notation /24 to indicate that the IP subnet prefix consists of 24 bits. Another way to say the same thing is that the subnet mask is 255.255.255.0. This is a common net mask to use.

There are other ways this might be done, but I think this should be the most straightforward. For example, you might also set this up as a single IP subnet, in which all of the wireless LANs belong to the same subnet, with the same IP address prefix. Just one big layer 2 network.

Can you please be more specific with your problem? 

i.e. The WiFi Network and devices you are using.

Router model you want to add, IP addressing scheme 

Existing Configurations of the WiFi Network

it will very helpfull if you post your network topology and the devices used

As other have said, you could try to be a little specific by providing more setup information because "WiFi module" can mean quite a number of things not limited to a module within a software app or just bare hardware. Are you simulating or emulating your lab environment?

When you buy a router, the description inside the booklet or at the back of the box will teach you some tips, you can also find it at the manufacturer website, alternatively, you should Google it too...basically, you need to search it at ICT shops, at online ICT shops, at university or service provider...

Being more specific in certain details like whether you are simulating or setting up real time routers, home or office purpose, kind and brand of routers will be more helpful for us to help you.

The following steps for this connection:

1)  Determine the IP address and the Subnet Mask of our MASTER router.

2) Open your web browser and type the router’s IP address into the address bar. If you are using IE, you might want to add http:// to the address else IE may throw an error.

3) Once you are inside the router settings, switch to Wireless Settings and make a note of the Wireless mode, the SSID and the channel.

4) configure SLAVE, our second router. First reset the router to factory defaults by hard-pressing the reset button with a pin for about 10 seconds.

5) Open the browser again and type 192.168.1.1 which is likely the the default internet IP address of your SLAVE router. If you are not using a Linksys router, try 192.168.0.1 which is the default IP for Netgear and D-Link routers.

6) change the IP address of the SLAVE router to any free address in your LAN. For instance, if the IP address of MASTER router is 192.168.30.1, you can safely assign a fixed IP address of 192.168.30.2 to SLAVE. Also make sure that the Subnet mask is the same as determined in Step 1. Save the settings of the SLAVE router and close the browser window.