You'd probably be better off compiling a new kernel rather than patching it. I'm not familiar with Mandrake 9, so I don't know what kernel it uses. Go to here
to download the latest kernel sources. And then you can follow the instructions here
If you insist on patching it, there are instructions in the second link on how to do it. I think you'd be better off learning how to compile a new kernel though. It's really not that difficult. Good Luck.