You DON'T!
I don't think you realize just how many packages are in portage.
You'd still be compiling them all a year from now
.
Also many of them are different choices of the same tool, and installing them all would be redundant, and in many cases impossible (i.e. portage won't let you install all of the different cron daemons).
If you want a massive no-thought install then gentoo probably isn't for you. With gentoo you pick and choose and customize only the packages you actually want or need.
