DD-WRT/"Flashing" Instructions

Via user interface
The router is to be connect with a LAN cable to the computer on which the firmware file is already loaded. The LAN cable on the router shall not be plugged into the Wan jack, but into one of the usually several LAN jacks.

After logging in, choose update by uploading the firmware file. The memory of the router does in some cases limits the size of the new firmware. During the update, the old and new firmware are on the router together, which can lead to size problems. Therefore it is recommended to load a small firmware at first and after that to update with the required firmware.

Source
The DD-WRT portal provide a router database. This database recommends firmware binaries for every suitable router. However, these binaries are not or hardly maintained and may contain bugs that have been known for a long time.

Until 2010, stable versions were regularly released. After 2010 there are many more updates, but these are only available as beta builds.

You can detect the propper firmware by reading the forums or by trying by yourself of the firmware in the download-center. For this, the router database can be of help in order to determine the sub-location for the appropriate firmware. But remember, that newer is not necessarily better.

What will brick the router

 * Flashing with too large firmware file. See above.
 * Flashing with an older revision which is first supported for your router. Compare the router database.
 * Flashing if router does not support 2.6 version.