Kernels/Arch Build System
The Arch Build System can be used to build a custom kernel based on the official package. This compilation method can automate the entire process, and is based on a very well tested package. You can edit the PKGBUILD to use a custom kernel configuration or add additional patches.
Getting the Ingredients
# pacman -S abs base-devel
First of all, you need a clean kernel to start your customization from. Fetch the kernel package files from ABS:
$ ABSROOT=. abs core/linux
Then, get any other file you need (e.g. custom configuration files, patches, etc.) from the respective sources.
Modifying the PKGBUILD
Modify the PKGBUILD of the official linux package.
Modify pkgbase=linux for your custom package name.
You can now proceed to compile you kernel by the usual command
If you have chosen an interactive program for configuring the kernel parameters (like menuconfig), you need to be there during the compilation.
After the makepkg, you can have a look at the linux.install file. You will see that some variables have changed. Now, you only have to install the package as usual with pacman (or equivalent program):
# pacman -U <kernel_package>
Now, the folders and files for your custom kernel have been created, e.g.
/boot/vmlinuz-linux-test. To test your kernel, update your bootloader (/boot/grub/menu.lst for GRUB) and add new entries ('default' and 'fallback') for your custom kernel. That way, you can have both the stock kernel and the custom one to choose from.