If the new kernel fails to boot, you can reset the PRAM (on PPC-architecture) by pressing command- option-p-r. Hold the keys down, until you have heard the boot-chime three times. Another method is to boot into OpenFirmware by pressing command- option-o-f upon start up to change the value of boot-file directly. If the new kernel booted just fine and you decide to install it in the place of the regular kernel, you can rename it in / to mach_kernel:

% sudo cp /mach_kernel /mach_kernel.dist
% sudo mv /mach_kernel.test /mach_kernel

To make sure that the kernel and kernel extensions are loaded individually and don't come from prelinked and cached copies, the mkextcache needs to be invalidated. This is done by updating the timestamp on /System/Library/Extensions:

% sudo touch /System/Library/Extensions

On the I386-architecture it is the best to boot using the install-cd, use the option spawn a shell and mount the root Volume in order to edit the com.apple.Boot.plist file.