How to build a custom rt kernel

I need a patch on alsa so I thought I could use the source to build a customized rt kernel (I'm using ubuntu studio).
When I copy /boot/*config to .config and create a kernel package it gets surprisingly big - and does not work (complains about a unknown vfs block).
So I assume I'm doing something awfully wrong. Could you give me a hint how to build a customized kernel - similar to the one shipped?