Follow these steps:

1. 1. Download the patch from this page.
   

2. 2. Have your Linux kernel source handy.  You can download it here.
   

3. 3. Copy the patch to the top of your Linux kernel source tree.
   

4. 4. Apply the patch, using:  patch -p1 < linux-2.6.20.3-ext3cow.patch
   

5. 5. Configure the kernel to use ext3cow.  (make menuconfig.  Under “File Systems”)
   

6. 6. Compile and install the kernel.
   

7. 7. Done.