To flash, use the flash script if possible (preferred) or follow the instructions on the pages.

Sometimes, you will only have success with this method (contributed by Swen “ssimon” Simon, thanks!):
• it seems to only work with atftp on the command line
• have the device powered off
• start atftp put, try this command line from ssimon:
atftp --trace --option "timeout 60" --option "mode octet" --put --local-file "WRT54GSV3.0_4.70.8_ETSI_code.bin" 192.168.1.1
(insert “freewrt-linksys-wrt54g*-brcm-*.bin” for the filename though)
• press reset button, hold it pressed
• power on the device only now
• wait until the firmware is uploaded
• release reset button only now