I've never tabbed, and so far I haven't needed to. Every song I've wanted to practice (haven't nailed any of the songs down, unless you count Christmas carols and the like) has been done. But now I wanna learn to play Descending by Saturnus, which no one seems to have done. But I don't know where to start... I'm assuming the song is probably above me and my ear.

Could you maybe suggest some easier songs to tab that already have tabs for me to grade against, so I can come back and nail Descending?
Just listened to that song and it's actually a pretty good type of song to start out with because it's slow paced (except some parts with faster notes) and the instruments stand well out in the mix. The problem though is that it's quite long and that the lead has a lot of variation, which together makes it a lot of effort to tab - and as a beginner, I guarantee this will take a ton of hours to get decent. I'd suggest you find a short song, about 3 minutes long or less that has an easy and slow melody. I started out tabbing similar music to this myself and it's some of the easier stuff to tab.

It's actually a good idea to try tab something that already has a good tab, then you can look at the other tab later and discover many things you didn't hear
If you're gonna tab, always choose a song you really love - because you're gonna be listening to it over and over dozens of times. Therefore it's hard to recommend songs.

Find some software (like Audacity, Transcribe, even VLC) you can slow down tricky parts with, it will really help - and if you can play along with your guitar (with a similar tone to the song you're doing) while listening - that's gonna be of great assistance as well.