You will need a line such as this in your make.conf:
PORTDIR_OVERLAY="/usr/local/portage"
Then, as root:
# . /etc/make.conf
# mkdir -p $PORTDIR_OVERLAY/net-misc/klapjack
# cp klapjack-x.y.z.ebuild $PORTDIR_OVERLAY/net-misc/klapjack/
# emerge klapjack
!!! No package digest file found: /usr/local/portage/net-misc/klapjack/files/digest-klapjack-0.4.2
!!! Type "ebuild foo.ebuild digest" to generate it.
# ebuild $PORTDIR_OVERLAY/net-misc/klapjack/klapjack-x.y.z.ebuild digest
>>> Generating digest file...
<<< klapjack-x.y.z.tar.gz
>>> Generating manifest file...
<<< klapjack-x.y.z.ebuild
<<< files/digest-klapjack-x.y.z
>>> Computed message digests.
# emerge klapjack
Compare the generated digest files with the MD5 values displayed on this page if you want to make sure the tarballs are Ok.