The installation procedure is the same as for the old NanoStation 5 but you have to use this image:

http://downloads.openwrt.org/backfire/10.03.1-rc5/ar71xx/openwrt-ar71xx-ubnt-nano-m-squashfs-factory.bin

Power cycle the device while you keep the reset button pushed. Leds will start to blink red and yellow.

Then tftp the image to 192.168.1.20