1. Download the mainline kernel that matches the kernel you want to patch, 3.0.20
2. Download the latest mainline kernel, 3.0.21
3. Extract both into the same directory
4. In that top directory, diff -Nur linux-3.0.20 linux-3.0.21 > 1.patch
5. Now in your main kernel source directory, patch -p1 -f --dry-run < 1.patch
6. Double check for any fails, you'll have them, unless you've patched the kernel close to mainline
7. Do the patch for real, patch -p1 -f < 1.patch