To use the packages, I recommend the following steps: First, create a file /etc/portage/binrepos.conf with the following content:
You can pick a different mirror according to your preferences (but also see the remarks below). Then, edit /etc/portage/make.conf, and add the following EMERGE_DEFAULT_OPTS (in addition to flags that you might already have there):
And that's it. Your next update should download the package index and use binary packages whenever the versions and use-flag settings match. Everything else is compiled as usual.