Installing Tomato Firmware on an ASUS WL520GU wireless router

Most of the websites teach you the old way of installing the Tomato firmware on an ASUS WL520GU wireless router, among them a guide list here is one of the best . Be sure to get the new password if you follow this guide after you installed the DD-WRT firmware.
If you want to install Tomato without installing the DD-WRT firmware first you can follow this guide.

Installing from original OEM firmware

1. Check the version of the firmware by  typing http://192.168.1.1 in your web browser.Default user/password is admin/admin. If the device has OEM firmware version 3.x or later, then you need to revert it to a pre-3.x version.
2. Download a pre-3.x version firmware from support.asus.com.tw, like version 2.0.0.7.
3. Go to http://192.168.1.1 find System Setup/Firmware Upgrade to upload the downloaded firmware.
4. Downloading and unpack any version of kernel 2.4 Tomato firmware, rename the file to "WL520gu_2.0.0.9_EN.trx". If you want to try the Kernel 2.6 version of the Tomato firmware, make sure you choose “Lite”.
5. Repeat step 3, to upload the Tomato firmware.

(Credit to original author, link)