I use genkernel to install my kernel.. this isn't the best way to do things but for now it gets the job done. Gentoo isn't my primary distro so I sometimes forget the steps to go through when they push a new kernel.

Make sure Gentoo is matched up with your target kernel version

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eselect kernel list
eselect kernel set #

Build it

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zcat /proc/config.gz > /usr/src/linux
cd /usr/src/linux
make oldconfig
make modules_prepare
emerge --ask @module-rebuild
make
make modules_install

Install it

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genkernel --no-menuconfig --no-clean --install all

Update bootloader

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vim /boot/extlinux/extlinux.conf

Reboot and cross your fingers.

Clean-up/delete old files in /boot and corresponding bootloader entries.

Useful Links and References

1. Gentoo Wiki: Kernel/Upgrade