The Installation of OpenWrt is quite device specific. A detailed procedure should be found on the device-pages in this wiki: Table of Hardware. If none is found you should read on here. Definitely have a look at the →Flash Layout.
If your attempt to install OpenWrt fails, generic.debrick could help you.
This HowTo is VERY generic. You cannot just use it, but you have to adapt values and other stuff to you hardware
Generally, you have three options:
Option_1:install OpenWrt onto the router's non-volatile memory: this page
Option_2:install OpenWrt into the RAM:
There are four ways to install OpenWrt on a device:
Open the WebUI of the original firmware with your web browser and install the OpenWrt firmware image file using the "Firmware Upgrade" option of the original firmware. Done.
NOTE: Sometimes the upgrade functionality of the original firmware contains some mechanism to make sure you do not inadvertently flash a wrong firmware-file. If that is the case, it will prevent you from installing OpenWrt this way.
To facilitate exactly this, namely to allow flashing of a firmware, most if not all bootloaders contain a build-in functionality for this purpose. Some contain a TFTP-client, others a TFTP-server, others a FTP-client and some even a FTP-server. You need to know:
Once you know all of the above parameters, you may proceed:
NOTE: Sometimes even the bootloader prevents you from flashing a non-OEM firmware, e.g. the Inventel DV4210 (AKA Livebox)
Time window: To be able to use the entire time window, the connection between host and router needs to be established quickly. To make this as quick possible, you could disable auto-negotiation of your NIC and especially disable media sensing of your OS.
This step shows you howto upload a new ramdisk image to the device. The existing firmware on the flash remains unchanged! You require a working serial cable to do the ramdisk upload.
./wndr3700 openwrt-ar71xx-uImage-lzma.bin openwrt-fixed.out
setenv ipaddr 192.168.0.1 setenv serverip 192.168.0.10 setenv bootargs 'board=WNDR3700' tftpboot 80800000 openwrt-fixed.out bootm(if you forget the bootargs piece below, the board will boot and look normal, but it won't be able to bring up any of the network interfaces!)
Concrete examples: wnr2000, …
Don't forgett to consult the other Generic Basic Howtos for OpenWrt.