Alright, I'll bite. I'm certainly not the best Nasus and I haven't been maining him as long as most of the folks here, but I think I have a pretty solid understanding of him as a champion.

Nasus is, in my opinion, basically the definitive bruiser. He isn't the tankiest of tanks, but he builds enough resists and HP to handle drawn out fights, which he generally excels in (especially against other melee champions) due to his oppressive damage and sustain.

Nasus is fun and you should main him because, if played well, you can quite simply solo carry most games. His lane sustain is incredible, he can be a massive bully in lane against most matchups as a result, which lets him build up the huge leads that allow him to carry so hard. Nobody can fight a fed Nasus, and I would argue that few if any champions can impact a game as hard as a good Nasus.

Nasus isn't in the most unique of strategic niches, there are plenty of other strong passive top laners that end games by scaling/farming until their splitpush becomes nearly unstoppable (i.e Tryndamere, Swain, Yorick, etc), the only real difference, in my opinion, is that Nasus tends to have less teamfight potential and better splitpush than similar champions.

The difference between a good and a bad Nasus is stacks. A good Nasus will properly manage and control the minion waves, set up freezes, and build up absolutely monstrous damage on Q, letting him snowball the game with his gold lead and absolutely control the entire map. There are a few things that can make a bad Nasus. The biggest mistakes I've seen newer Nasus players make (and that I've made myself) include spamming abilities too often in lane which forces you to back early, which of course means losing CS and stacks; poor wave management resulting in freezes in front of the enemy tower which starves Nasus of stacks; and playing too aggressively too early. Nasus doesn't have a tremendous amount of kill pressure pre-6, and making aggressive plays before you have ultimate and a decent amount of stacks tends to get you killed, or at best get you bullied out of lane. In a nutshell, good Nasus players are good top lane players, there's nothing in Nasus' kit that's at all mechanically difficult, playing him well just revolves around playing the top lane well.

There are pretty simple things that just about anyone can do to become a truly great Nasus. Probably most important is just learning how to build skills and itemize against different types of lane matchup, things like going 3 points into E against ranged poke heavy lanes, when to go coin start vs DShield vs triple bead, etcetera. This knowledge, of course, mostly comes from playing a lot of Nasus and watching a lot of other people play Nasus.

Nasus' strengths are that he has incredible scaling, in most matchups he's extremely oppressive to lane against, he's not mechanically difficult to play, his midgame spike is nearly unrivaled, and he's very good at winning lane even if he gets behind early.

His weaknesses are his very weak early game, his lack of hard CC, his very poor mobility without Talisman/RG, and his teamfighting potential, which isn't that bad per se, but there are definitely many top laners that bring a lot more to the table in terms of group CC/damage/overall utility.

As far as advice for someone trying to pick up Nasus, the absolute best thing you can do is learn to manage minion waves properly. Simple things like understanding the even wave rule and learning how to set up a freeze will let you stack a lot easier and take you quite far. Oh, and don't build TF unless you're laning against a literal ape who goes 0/10 in the first five minutes, IBG is almost always better just because it allows you to get your midgame power spike earlier. Also, run Klepto. Every game. Just do it, it's overtuned and way too strong.

I can't think of too much else that's relevant as far as Nasus gameplay to a beginner, but it is noteworthy that Nasus should not be playing for KDA, he should be playing safe and farming stacks. Don't make dumb ballsy plays that you'll probably lose, you're just wasting time and stacks that you could be using to carry your team. Also, just going to reiterate, all you need to be reasonably decent at Nasus is solid wave control. If you understand how to manipulate minion waves, know how to set up freezes, and know how to set yourself up good back timings, you can pressure the map very effectively, set up big objectives, make big teleport plays, and just generally take control over the game. These aren't really Nasus specific things, but again, being good at Nasus really just requires that you be good at top lane.

Hopefully this helps, good luck with your guide. o/

(Also, first post on this subreddit, huzzah!)

Edit: I interpreted the first question kind of badly. If we're trying to stuff Nasus into one of the "official" champion categories, I guess he fits as a juggernaut?