It is advised by the developer that the process is carried out on a Linux-based machine.
Step 1: Download DD via this link.
Step 2: Download the required custom ROM for Lumia 800 or 710, depending on which phone you have, from the official thread on XDA here.
Step 3: Make sure the device is connected to your computer.
Step 4: Next, you’ll need to locate the appropriate partition:
dd if=./os-new.nb of=/dev/sdX9(note that ‘x’ is the disk detected by your Linux distribution).
Step 5: Now, you ll need to perform a hard reset of the device. In order to do this, hold the power button for 10 seconds (which will take you out of disk mode), then press and hold power + volume down + camera until the device begins to vibrate once again. Next, release the power button while still holding volume down + camera for around five seconds or so in order to complete the hard reset.