Just went through my notes and found that building a custom kernel on #FreeBSD is really simple. Same goes for editing the actual kernel config file.
Basically:

- Fetching the source
- tar -C / -xvf src.txz
- cd /usr/src/sys/amd64/conf
- sudo cp -v GENERIC CUSTOM
- sudo vim CUSTOM
- cd /usr/src
- sudo make buildkernel KERNCONF=CUSTOM
- sudo make installkernel KERNCONF=CUSTOM