The Linux Kernel is just another recipe for Yocto, so learning to patch it you learn to patch any other package. In the other hand, Yocto **should not** be used for package development, but in those rare cases follow the below steps. It is assumed that you have already build the package you want to patch.
1. Create the patch or patches. In this example we are patching the Linux kernel for [wandboard-dual](http://www.wandboard.org/) machine; in other words, the value of MACHINE on the `build/conf/local.conf` is `MACHINE ??= 'wandboard-dual'`. In case you already have the patches, make sure these can be nicely applied with the commands `git apply --check <PATCH_NAME>`, and jump this step
build $ cd tmp/work/wandboard_dual-poky-linux-gnueabi/linux-wandboard/3.0.35-r0/git
build $ # Edit any files you want to change
build $ git add <modified file 1> <modified file 2> .. # Select the files you want to commit