Learnt with pain:
One of the most important things if not nearly the most important thing in installation of gentoo by a person not considering himself a guru in linux is configuring the bootloader. The reason is very simple. Migrating from other OS, you most probably would like to keep your stable working operating system for a little bit while messing with gentoo. The installation is sometimes long and quite painful process along which any simplest problem might prevent you from actual using gentoo for everyday operations. Example would be an issue with a graphics driver which would not allow you to properly configure xorg and therefore you would have to use console for a while. I don't know about you, but personally I can't use Links and alike to check my email. My eyeballs turn red, nose starts bleeding and all that bad stuff. And because checking e-mail is something so ridiculously important for a University student nowadays, I would need a working X. The workaround for this particular problem is just using LiveCD which has a guied browser. But the point is understood: you would want to keep another working OS unless you install gentoos everyday like crazy and can do it without a monitor! Hence, you should always have a properly configured boot loader.
Here I want to illustrate how bad it is to underestimate the importance of configured bootloader.
It was winter of 2008-2009, the winter break at college so I went home. I was an alltime Ubuntu user at the moment and (as mentioned in earlier posts) decided to try Gentoo. I got to the point (about that in the next post) where I had working gentoo with no Xorg and no network. The xorg part at that point was just the matter of learning how to configure it manually. For network - somebody should have told me about DHCP!!! Anyway, I basically couldn't do anything on it (when I said "working gentoo" I meant the kernel was actually loading :-D). The computer at home had Windows (for mama) and Ubuntu for myself. I erased ubuntu during the process of gentoo installation, but of course the grub was still there! And since it was configured long ago automatically by Ubuntu, ...nothing loaded. The only possibility for me was the LiveCD on which I pretty much lived for quite a bit. The most painful part was when my mom came over once and said "I need to check my email". It was very hard to explain a 40-year old piano teacher why the computer could only produce green text on the black screen and why in the hell that "was ok". I loaded the livecd but it turned out she had all the passwords saved on her windows. For a month I was claimed to have broken the family computer and somehow erasing all of the "collected for long time" by real careful users' passwords. I made her happy when I finally figured out GRUB and could eventually load her windows WITH ALL THE PASSWORDS ALREADY THERE!!! So much joy...
Advice from a noob:
Guys on Gentoo.org give a really nice description of GRUB and guidelines of how to work with it in the handbook. Pay attention to that, go to different sources if it doesn't make sense, look up the grub's config on your distro, but you have to know your grub!