There is some info in that Tutorial which isn't exactly correct however most of the information is still useful.
You will need to configure the IP Address of the second and/or additional router to be in the same Subnet of your first Router. If your main Router has a DHCP Range of 192.168.1.2 to 192.168.1.50, you may assign a Static IP of 192.168.1.3, 192.168.1.4 and so on….to another router that you will be adding to your Network.
To correct this information, you don't assign static IP's that are in the range of DHCP addresses. Notice how the assigned static IP's fall into the range of DHCP. These should be outside the DHCP range, for instance starting from 192.168.1.51 and up should be used for static IP's because it's just outside the range of DHCP addresses being used by this router.
If you need to use 192.168.1.2 and up for the A/P's then you can simply adjust the DHCP range in the routers settings, say move it from 192.168.1. up too 192.168.1.5, this leaves room for the static IP's at the bottom of the range. If you don't do this then the routers DHCP could assign addresses that are already in use which is a common problem, saw this happen just last week.
This next paragraph made no sense to me at all.
3. On the additional router(s), setup the wireless configuration which is identical to the current or main router. You may use Wireless Channel 1, 6 or 11 which have very good signals. The Network Encryption (WEP, WPA, WPA2) will need to match the SSID (wireless network name) of the main router.
First of all the you can have a different SSID on each router if you want, they don't need to be the same although moving from one to the other would be more seamless that way because it would auto connect to the best signal. Sometimes it's better to have two different SSID's for the A/P's for various reasons. You have some flexibility here.
OK, the encryption has nothing to do with the SSID "name of the network" I'm not sure what they mean by "the encryption needs to match the SSID"?
Then they say encryption is the security which it's not, it has the two mixed up as they are two different things, security is WPA2 where encryption would be TKIP or AES.
It might be worth mentioning that TKIP isn't made to work at wireless N speeds. AES is made to work at wireless N speeds, both are encryption methods where security is WPA2 etc.
There is also more to channel selection than just choose 1, 6, or 11 but that's another subject really.