From DD-WRT Wiki
!!!ATTENTION!!! This guide was copied from my guide posted on the forum. This guide is ONLY for the WRT54G-TM router, NOT for the WRTU54G-TM. Do NOT attempt to use this guide to flash your WRTU54G-TM router.
 How to flash the Linksys WRT54G-TM T-Mobile Edition by Luniz2k1
In order to flash this router you need to do the following steps:
- Download the DD-WRT firmware (MUST be v24-RC5 or newer (index) - v24-SP2 build 10404 is suggested)
- Download the TFTP utility
- Download Tornado's CFE_Updater-WRT54G-TM.bin
1. The default LAN IP Address of the Linksys WRT54G-TM is 192.168.0.1. You will need to set a Static IP Address on your computer for this procedure. If you are on Windows I suggest that you reset the router to it's defaults and set 2 LAN IP Address on your network controller to speed up the process, otherwise once you upload the CFE_Updater-WRT54G-TM.bin you will need to change your LAN IP Address from 192.168.0.2 to 192.168.1.2 to be able to TFTP DD-WRT onto the router.
On Windows XP, Control Panel/Network Connections/right-click on Local Area Connection icon and select Properties. On Windows Vista, Control Panel/Network and Sharing Center.
Go into the properties of the Local Area Connection/Internet Protocol TCP/IP and set the static IP address.
IP Address: 192.168.0.2 Subnet Mask: 255.255.255.0 Default Gateway: 192.168.0.1
Click the Advanced button in the TCP/IP setup window and in the IP Address section click the ADD button to add the following 2nd IP Address to the adapter:
IP Address: 192.168.1.2 Subnet Mask: 255.255.255.0
Then click OK on each screen. Your network interface card (NIC) now has 2 LAN IP Address statically set on it, 192.168.0.2 and 192.168.1.2. This will allow your PC to communicate with the router before and after replacing the CFE on it. It will also allow you to be able to tell when the router is ready for a TFTP upload.
2. Open a command prompt window and type ping -t 192.168.1.1 and hit enter. Leave this window open.
3. Log into the WRT54G-TM's web interface @ http://192.168.0.1 (assuming that the router has been reset to defaults) and go to the Administration Tab - Firmware Upgrade. Upload the CFE_Updater-WRT54G-TM.bin to the router.
!!!ATTENTION!!! You MUST wait at least 5 minutes after clicking the Upgrade button to allow the CFE_Updater to replace the stock CFE on the router and for it to erase the rest of the flash, otherwise you will brick your router.
A few seconds after clicking the Upgrade button the Linksys web interface will report that the upload has been successful and the power light will begin flashing on the router. Do NOT assume that the CFE replacement/flash erasing/reboot process has finished as the power light will continue to flash after it has rebooted. So there is no way to tell when it is ready for you to TFTP the DD-WRT firmware to it except when you begin to see the ping respond.
So once it is finished replacing the stock CFE and erasing the rest of the flash, the router will reboot. When the router is ready for the TFTP upload of the DD-WRT firmware, you will see the ping -t 192.168.1.1 begin to respond in the command prompt window that you opened before.
4. Start up the TFTP utility and set the following:
Server: 192.168.1.1 Password: <leave blank> File: <path to dd-wrt.v24-10404_NEWD_mega.bin> (or whichever build you wish to use)
Now click the Upgrade button. Once the firmware is sent to the router, it will reboot. The power light will change from flashing to solid when it is ready to be configured @ http://192.168.1.1
A few notes:
- Tornado has made it so that the router will retain it's MAC Address that is on the label on the bottom of the router with the new CFE replacement.
- Tornado is in the process of making a revert bin for those who may choose to revert back to the stock Linksys firmware (god only know why Cool )
Many thanks goes out to Tornado, Eko, and Brainslayer for their work on this!!! Cool
No more will you need to void your warranty by opening the WRT54G-TM to JTAG DD-WRT onto it, or spending $70+ USD for a WRT54G-TM pre-loaded with DD-WRT from eBay Cool
 Using a Mac by StevenJJ
I just followed the instructions on Mac Leopard, using MacTFTP (http://www.mactechnologies.com) and it worked very well.
Some tips you might want to try: Use Camino, it is more compatible! To get two manual IP addresses on your Ethernet port in Leopard, in System Preferences/Network, create a new Location (call it wrt54g-tm), minus the ports you won't be using to get them out of the way, click + to create another Ethernet, it will come up as Ethernet2. Set one of them to manual 192.168.0.2 (and the rest) and the other 192.168.1.2 (and all the rest as described above for Windows).
Start Terminal, Enter ping -t 600 192.168.1.1 and move it aside.
Go ahead and flash the CFE firmware as above, and when the Terminal window finally shows successful returns to the ping, start MacTFTP Client set just as recommended for Windows. Quit terminal, it's not needed anymore.
Be patient (I went out for dinner) when it is finished you'll have a router ready at 192.168.1.1.