This guide is for flashing DD-WRT v24 RC6 or earlier firmware only.

Visit the forums to discuss this guide.

Prep 

The DD-WRT firmware can be downloaded here.
Once you're there, choose the version you would like to download.
Download "root.fs" and "vmlinux.bin.l7" and save them to a folder like C:\tftp

You will also need a tftp program to host the flash images for the Fonera.  I recommend using tftpd32 which can be downloaded here (free).
Once you have tftpd32 installed, make sure the "Current Directory" points to the folder you saved the flash files in the step above.  Also take note of the "Server interface" IP address as well.

 Redboot will only look in the root folder of the tftp directory, so put the files in the root directory.  E.g.  put the files in C:\tftp not C:\tftp\flashfiles

 Installation

1.

Access Redboot.

Specify the local IP and tftp server IP address.  Use the following command:

ip_address -l 192.168.1.254/24 -h [remote server address]  

Use the IP address of the tftp server you setup earlier.  For example, the tftp server IP address in this instance is "192.168.1.161".  Therefore, the command would be

ip_address -l 192.168.1.254/24 -h 192.168.1.161

Ignore the "/home/alfa..." messages if you get them.  They are harmless and the flash will work nonetheless.

3. 

fis init 

4.

 load -r -v -b 0x80041000 root.fs

 Run the command:

fis create -b 0x80041000 -f 0xA8030000 -l 0x002C0000 -e 0x00000000 rootfs

This process can take 10-20 minutes.  In this case, it took just about 15 minutes.  After you enter the command, there will be no output on the screen until the Fonera is done.  Leave it alone until it is finished! Do not power it off or disconnect your session!

 
load -r -v -b 0x80041000 vmlinux.bin.l7

 
fis create -r 0x80041000 -e 0x80041000 -l 0x000E0000 vmlinux.bin.l7

This process may take some time like in Step 5.

8.

 
fis create -f 0xA83D0000 -l 0x00010000 -n nvram 

You're done! Use the "reset"command to reset the router, or just power it off/on.  Once it restarts, you should be able to see a "dd-wrt" access point.  Connect to it and enjoy your new router.