From DD-WRT Wiki
This Wiki article is a Work in Progress.
This router REQUIRES a K26 build. All of the K26 builds have "_K2.6_" in the filename. If you need a firmware recommendation refer to the Firmware Recommendations thread and make sure to get a K26 build. Also be sure that you don't use a mega build otherwhise it will brick your device.
 HIGHLY recommended reading
Peacock Thread FAQ Especially the "Required Reading" notes listed at the beginning of the thread
Redhawk's Firmware Recommendations
 Which Firmware?
First you need to know that this router only supports the "NEWD-2_K2.6" firmware. Next, you need to know which revision number (i.e. build 13525) you want to use. For recommendations on what revision to use don't trust the Router Database. Instead you should first read the Peacock Thread keeping in mind that this router REQUIRES K2.6. Pay particular attention to Note 3. Then read the Firmware Recommendations Thread (still keep in mind K2.6). You should now have a pretty good idea of what build number you want.
Now you need to know what build version you want. Compare the features of the different build versions here. This router has 8MB of flash memory so it will support any of the builds in that list. You probably want to get a build version that supports USB since this router has a USB port. Warning: Mega builds will brick your device.
Next, a question: Are you flashing from a stock router with Netgear firmware? If not then you can skip this paragraph. If so then first you need to flash with a special build called a "trailed" build. A trailed build ...
... Some more stuff ...
Need to re-word the following and take out the mention of the Router Database... Are you flashing from the original Netgear firmware? The Netgear firmware won't recognize a standard build of DD-WRT. You need to start with a special build that the Netgear firmware recognizes. You can grab one from the Router Database. Make sure to grab the one that says "Special File for initial flashing". It should have "_NEWD-2_K2.6_mini-WNR3500L.chk" in the filename. After you flash this initial image you should have a working version of DD-WRT on your router. But wait! You're not done yet. This initial version doesn't have very many features and is probably not a recommended build. (Don't trust the Router Database recommendations) Also, the WNR3500L has an 8MB flash and 64MB RAM so it will run even the "Big" builds with room to spare. Don't hesitate to use at least the "Standard-USB-FTP" build.
Outdated, needs to be removed.
 Installation of DD-WRT
I’m sure someone will ask about this. Here’s the deal, straight and to the point:
- The WNR3500L requires a NEWD-2, K2.6 build of DD-WRT
- You will first need to upload a special .chk build (also K2.6) of DD-WRT to the router to go from Netgear’s factory firmware. Then you may install a .bin build of DD-WRT.
- Spend some time browsing the DD-WRT Forums http://www.dd-wrt.com/phpBB2/ There’s a lot of chaotic information to ingest.
For a brand new WNR3500L, installation goes like this:
- Set a static IP such as 192.168.1.8 on the PC you will be using for the flash.
- Connect the PC to the router (unplug everything else!)
- Perform a 30-30-30 Reset http://www.dd-wrt.com/wiki/index.php/Hard_reset_or_30/30/30 on the router.
- In Internet Explorer, go to http://192.168.1.1
- Cancel the router’s automatic firmware update checking, then click on “Router Upgrade” in the menu at left.
- Upload the special .chk firmware file.
- Wait 5 minutes after the router says it is done, then perform another 30-30-30 reset. Make sure you clear your browsers cache between updating firmwares.
- Repeat steps 4-7 with the .bin firmware file.
- Login and start tweaking
These are the exact files I used:
Credits go to: blackinches
 DeBricking the router
 Reverting back to factory firmware
For now this is a WIP, but in the interim, you can simply follow this guide to revert back...