What’s the process look like?

On openbsd it's

    cd /sys/arch/$(machine)/conf
    cp GENERIC CUSTOM
    vi CUSTOM    # make your changes
    config CUSTOM
    cd ../compile/CUSTOM
    make

I have never done it for 2bsd but according to http://www.vaxman.de/publications/bsd211_inst.pdf

    cd /usr/src/sys/conf
    cp GENERIC CUSTOM
    vi CUSTOM
    ./config CUSTOM
    cd /sys/CUSTOM
    make