Hi Jeff & Welcome! I'm fairly new here, myself....
Ahh the Mac12. It, too, was my first single malt... Long before I really
got into them. Absolutely wonderful, as is the Double Wood. Since then I've found a deep love for the wide varieties available (though I tend to truly savor the smoky Islay Malts).
The Classic Glenmorangie is indeed a huge step away from The Macallan 12. Life-maturation in sherry or bourbon makes a huge difference across the board - & so you're looking for more strickly sherried whiskies, Highland Park is notorious for their strict use of first (and second?) fill sherry casks... Though I can't say the HP12 would be anything quite like The Macallan 12 - as it harbors a much stronger finish among other reasons - but the HP18 (~$100) was the smoothest whisky I've ever had the pleasure to sample.
As far as nosing, tasting, & assessing goes - It's just a skill that develops over time. I have an WSET intermediate level certification, and I found that my previous concepts of aesthetic & tasting were only re-enforced & expanded by my professional training...
You simply ask yourself: "What do you smell, what else do you smell? What about when you smell it again? And Again?" On deeper levels, "What does it remind you of? How does it make you feel? Or, at the core of aesthetic appreciation, "Do you like it? Or not? And Why?!
Basically the same is applied to each aspect of tasting; Nosing, Palate (or Taste), and Finish.
Palate consists of things like sweetness or dryness (The majority of Scotch - depending on the heavy use of sherry, port, or first-fill bourbon casks - is typically on the drier side), "viscosity" or mouth-feel; Creamy, Watery, or "Oily" (like a Brandy) for example, as well as general flavor descriptors, among others.
Finish, fairly obvious, consists mainly of the flavors that linger in your mouth after you swallow, as well as the length
of their linger. Some may also call the actual act of swallowing (the "Smooth or Harsh" factor) part of the finish, as well.
Of course, you don't necessarily literally ask yourself these questions while you're in the process of a tasting... Often things just jump out at you. From there, I find it takes a bit more concentration to really hone in on particulars... I'm certainly still learning myself!
One important thing to assess throughout every stage of a tasting is Intensity.
Does the smell jump out of the glass? Does it taste rich & full in your mouth? Do you feel like you're drinking your breath after you swallow?
Another huge thing to factor in is familiarity. Perfect example: Heather. I've never smelled Scottish Heather, or had Heather Honey in my life. But it's a frequent descriptor in reviews. There are loads of other examples, but the moral is: smell stuff & taste stuff. A great idea (which I've yet to do) is to spend some time (and money) in the supermarket smelling exotic & regular fruits, pick up an assortment of nuts & sample them separately or sparingly to isolate flavors. Being familiar with tastes & smells is the number one way to pinpoint those notes that you're looking for in your whisky.... Though I did smell "fresh shower curtain plastic" on a bottle of French wine from 1967 at our Christmas Party, so don't limit yourself! Typical Islay whiskies will have notes that range from smoke & tar to ashtray... I've even heard "band-aid" and all sorts of other descriptors, so just remember: everyone is different & your notes will almost always be a bit different than someone else's tasting notes!