I have had the same question (though I didn't get a free router). Sad to say, the info I have found on this site is not very clear and/or assumes you are already know what to do. There is also an assumption that you want to rip open your router and attach an RS-232 connection.

What I'll do, is post what I think is true and we can wait for someone to clarify.

Assume: stock out-of-box router with stock firmware. No desire to attach an RS-232 cable.

OK, so you need to work from a web browser on your computer (preferably IE as Firefox has issues).

The stock Netgear firmware need a file in '.chk' format, so download the version that you find on this site. It's somewhat old, and necessary, but disposable

Use the web-based GUI to install, following the instructions on this site.

Once this somewhat-old version is installed, you'll probably want to immediately replace it with a newer version. However, dd-wrt's update GUI requires '.bin' format files which is nice because that's what is on their site.

Once the firmware is installed, you want to updated to the latest good version from http://www.dd-wrt.com/dd-wrtv2/downloads/others/eko/V24_TNG/.. This will be in 'bin' format, which you can install using the dd-wrt/router GUI.

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In summary, to go from Netgear stock firmware to dd-wrt, use the Netgear web GUI to install an older '.chk' version of dd-wrt. From then on updating from one version of dd-wrt to the next is done using the dd-wrt web GUI to install '.bin' files.

You can look at the following link to see how to istall DD-WRT on WGR614L or WGR614v8 .

http://www.myopenrouter.com/article/10339/DD-WRT-for-NETGEAR-WGR614L-Installa...

You can follow the similar steps for installing other open source firmwares also .

well I have flashed my router with the firmware avlbl at :
http://www.myopenrouter.com/download/10472/DD-WRT-v24-SP1-Build-for-WGR614L-W...

you can try this ...as it is an stable version and will not brick your router ....
Please note that this link only gives you the firmware .... the steps reqd to do the flashing are the same as those provided in the link mentioned in my last post ....