The manual for this process is: FreeBSD.org, Handbook ch. 5.
/stand/sysinstall, "Custom", "Distributions", "Custom", "Src", "sys". I also got "include" and "lib", I don't know if they're also needed, I just assumed so.
cp GENERIC your-kernel-file
'dmesg | more'to list the startup messages. For all devices that show up with a "not found" sort of error message, find the corresponding line in your-kernel-file, and comment it out.