Most of the time you can do it easily.

If the machine boots but you want to switch the kernel:

1: copy the new kernel to the root, but with a different name ie mach_kernel.voodoo
At the darwin boot prompt type without quotes "mach_kernel.voodoo -v

The added -v at the end starts the computer in verbose mode, showing you what is going on.

Then when your machine gets updates, you wont have to keep changing the kernel, because you are always typing that in the beginning. You can modify what kernel your machine starts from, but im sure thats covered somewhere...

If you are going to change it manually every time apple updates the kernel then:

1: go to terminal and paste w/o quotes: "defaults write com.apple.Finder AppleShowAllFiles YES"
2: in the root directory (finder) there is a file called mach_kernel, rename it mach_kernel old
If you cant rename mach_kernel, copy it to desktop, rename it, then copy it back to root, then delete mach kernel from root, thus making a space for the new kernel.
3: copy new mach_kernel to the root.
4: go to terminal and paste w/o quotes: "defaults write com.apple.Finder AppleShowAllFiles NO"

If you are doing this from a command line, ugh not my thing.
From windows, you will need a app that views mac disks, ie macdrive.

Today i switched from voodoo 9.5 to voodoo 9.7, and I am looking for anv kernels with decrypt. So if any one knows a working link (rapidshare is always too busy) plz msg me!