Building kernel and recompile?
 

I understand how to build a kernel but lets say you forgot something in kernel that you need.

Best approach?

I was thinking of
make mrproper
cp /boot/config-version /usr/src/linux/.config

or this not necessary?

is this correct approach? what about modules? lets say you want to remove a module from kernel, do you need to delete it from /lib/modules/kernel-version first?

To be honest, you are probably going to get a faster answer on the Gentoo forums. Not a lot of Gentoo users here. Just my observation.

you can just edit the existing config (via menuconfig or other) and then run make, make modules, make install, and make modules_install. it should only recompile the stuff that needs recompiling and not compile the whole kernel again. no need to start from scratch.

If you are already booting a working kernel, have the CONFIG_IKCONFIG_PROC option set and don't want to change anything, you can:
cd /usr/src/linux
zcat /proc/config > .config
make oldconfig
etc

Here is a really good video on how to update the Linux kernel. This is a good universal method for both new and old-comers
https://www.youtube.com/watch?v=Tn8U...channel=FIXAPC