I would like to practice installing Gentoo from scratch since I like the idea of the distro, but I’m quite new to Linux at all and only had experience with Debian. I suppose my very basic knowledge of terminal isn’t enough to install Gentoo, even with the handbook. So, what would you recommend to learn or practice before I actually try installing Gentoo? Also, any specific tips on installing Gentoo inside a VM?
As others have already noted, the handbook is very good. You should be able to manage with limited Linux & terminal skills. (terminal navigation, can install a package with e.g. apt, etc.)
If you’re unsure about your skills, try using the terminal a bit. You can start with switching to the apt cli (command-line program) if you’re using a gui client. Or, you could try to do some basic, everyday tasks with it, like editing text with nano (or vim, if you’re feeling brave).
Just don’t forget to install important packages when installing Gentoo, like sudo (and add youself to sudoers), a DE/WM (KDE Plasma, GNOME, Sway, etc.) and a terminal. These things can be done later, by rebooting to the install media and chrooting, but it can be a bit annoying.
Don’t worry too much about mistakes, as everything is fixable, except things related to bad UEFI implementations.
Like others, I would recommend going with defaults for now. You can always tinker after the install. While a few things are a real pain to change or shouldn’t be changed, like going from no-multilib to multilib, most things can be played with later. Though I haven’t done it with a Gentoo system specifically, going from an unencypted system to encrypting all disks is very much possible.