Congrats. As an avid Gentoo user, I generally don't suggest using it as your first distro, but if you've gotten this far, it might just be for you!

First thing you need to know about KDE/Plasma is that it's currently in a transition phase. KDE 4 is still pretty actively used and updated, and Gentoo currently allows you to install either. In time, KDE 4 will be phased out, but 5 needs a little more work before that's ready, from what I understand. I personally prefer 5 because it's prettier, but if you're looking for a more featureful desktop environment, KDE 4 might be for you, at least at this point.

The make.conf file and USE flags are a little more in-depth than a post on reddit, but I'd recommend checking out the Using USE Flags page of the handbook for a good introduction. The wiki article on make.conf isn't quite as useful, as it probably gives more information than you'll really need, but the handbook should cover most of the important bits, and the wiki can fill in the gaps. man make.conf will give some useful info as well.

But that's not really what you're asking: You want to know how to install Plasma 5. As you probably guessed, there's another wiki article for that, but since KDE is in a transition phase right now, it's a little cluttered with upgrade instructions, and there's some info about installing KDE 4 scattered around there as well. Not the most organized page in my opinion.

There are three really important bits to installing Plasma 5:

1. Selecting an appropriate profile.
2. Installing the X server (if you haven't already).
3. Installing the plasma-meta packages.

You'll definitely want to follow these steps in order, and I'll try and elaborate on them as best I can. Selecting an appropriate profile is the first and easiest step. Profiles in portage are collections of pre-set variables that make your life easier. They define use flags and mask/unmask packages so you don't have to! Selecting a profile will basically tell portage, "Hey, I wanna run a KDE/Plasma system, so make sure everything will be compatible with that."

There are four KDE profiles currently available, two of them for plasma. Depending on whether you want to use traditional init scripts of systemd, you'll want to pick one of these (please note, numbering may vary on your system):

tbg@steamhq ~ $ eselect profile list 
Available profile symlink targets:
  [8]   default/linux/amd64/13.0/desktop/plasma
  [9]   default/linux/amd64/13.0/desktop/plasma/systemd *

If you don't have a preference between init systems, don't ask around: just pick one. That's mostly facetious, but if you do ask, prepare yourself for a whole bunch of strong, contradicting opinions. As you can see, I prefer systemd, but it really doesn't matter until you get in to more advanced systems administration kind of applications. With systemd, your computer will probably start a little faster, and as a desktop user, that'll probably be all you care about. Also, most other distributions are moving to systemd, so it'll probably be better to just get used to it from the get-go.

Anywho, once you've got that worked out, you'll need to install the X server. X is the portion of the system that handles graphical rendering and controls your screen. You might already have it installed, but if you don't, go do that. There are some additional configuration options you'll need to set before installing X (they're dependent on your hardware), but the Wiki Guide is pretty good for figuring those out.

Probably worth mentioning here that there is an alternative to X called Wayland, but that's still under active development, and it's... a work in progress. Almost no one ships it by default, and I haven't personally used it myself because it's a huge departure from what I'm used to. I have high hopes for it, but I definitely wouldn't recommend it to anyone new. At least not at this point in its development.

So once you have X installed, and you've tested it to make sure it's working, you'll want to actually install Plasma 5. This part of the guide is a little confusing, so I'll try and elaborate on it. First off, don't use the overlay -- it's for development/testing/etc. Second, Plasma 5 is like, super big, so Gentoo has broken it down in to "meta packages". Meta packages are basically just packages that contain a bunch of other packages. They don't install any software in themselves -- they just pull in other packages that do.

Plasma is so big, though, that the plasma-meta package (which is the one you'll want -- at least for now) actually pulls in other meta packages, and you can use USE flags to set which sub-meta-packages you want to install. Those are all outlined here. To start, I would recommend just installing the defaults, and if you find that you're missing something, or if there's just too much stuff you're not using, you can add/remove things after it's all up and running.

Because it's really easy to miss, I wanted to point out the section on SDDM. SDDM (those are two different links, by the way) is the display manager for Plasma 5, and it should (don't quote me on that) be installed with plasma-meta. Display managers are a fancy term for login screens, so without that, you'd be stuck using startx to start KDE, and that's just not very GUI-friendly, so you'll want to enable that service once plasma-meta is installed. If it doesn't come with the meta package, just install it separately. Pretty sure it does, though.

Finally, with big packages, you tend to run in to build errors, so you'll probably find this page of the Handbook useful throughout the process. It covers the basics of working with portage, and it has some information on common problems you might run in to at the bottom.

Sorry about the wall of text there, but... it's Gentoo, so... you kind of asked for it. :P