Go to the directory with the source code:
cd freebsdsrc
Go to the directory with the kernel's configuration code:
# If your system is 32-bit. cd sys/i386/conf/ # If your system is 64-bit. cd sys/amd64/conf/
Get a copy of the GENERIC kernel (let's call it MODEDKERNEL). It will be the base of your customisations.
cp GENERIC MODEDKERNEL
Modify the MODEDKERNEL
file at your will.