The tip I heard from a real drywall finisher - put it on as thin as possible. Let it dry. Don’t keep working it (my mistake) the more you work, the more you pick up crud and lumps that leave gouges. So put on a thin layer, let it dry, and go back to fill in any bubble holes etc. The mud should be more liquid like pudding than like mashed potatoes (liquid also helps it smooth without work.) Less is more.
I have seen commercial jobs and professional homebuilders using sheets laid horizontally. This has 2 benefits. In an 8-foot high room, there will only be the one seam along the wall at stomach height, plus corners and ceiling corner. If they use 12-foot sheets, most will reach from corner to corner in smaller rooms. I assume it’s a lot easier to do a horizontal seam within easy reach than to do 8-foot high vertical seams. (For the ceilings they had strap-on stilts so the were within easy reach too.)
All the poly vapour barrier installs I’ve seen recently have a caulking sealant along every stud (and top, bottom plates). So hanging drywall with screws means the screw goes through the vapour barrier and also through the sealant into the stud - no leak issues. I don’t know that I’ve seen exterior walls with glued drywall for the very reason you mention. It’s more of a commercial technique I think, and I’ve seen it with prefinished drywall (a vinyl textured wallpaper finish) so no screw heads, and they had a push-in plastic strip that covered the seams.
Foil would be to prevent the heat from the furnace from overheating the (wood) walls and creating a fire hazard, I would imagine. If your furnace causes the nearby wall to get really hot, then the problem is not how to finish the wall - it’s that your furnace is probably too old and needs to be replaced. At very least, that’s a lot of wasted heat. (Or maybe the furnace was replaced but the wall was never updated. How hot does it get?)
This is all the kind of stuff that makes it challenging for a novice. Most other tasks, like installing a toilet–just involve a series of steps that need to be followed. But with joint compound, you have to get a feel for the consistency, how it flows as you spread it, how much pressure to apply, how to deal with low/high spots, etc.
Plus, mistakes may not be noticed until after the paint is applied and the light hits it just right. One reason for this is that the joint compound is very flat to the light and doesn’t show defects very well. Once you get paint which has some gloss, then the errors become much more visible. To avoid this, aim a very bright light along the wall so you can easily see the shadows from the defects. The glossier your paint is, the smoother you’ll need it to be.
It’s very doable and novices shouldn’t avoid it completely, but definitely be aware that there’s a learning curve. Don’t have your first try be in an area which is very noticeable.
Best way to check is to take a utility light (that light-bulb-in-a-cage) and hold it very near the wall of to the side of the joint. The shadows for highs and lows will be more obvious.
The other point Is that by putting on many thin layers one at a time after the previous dries, you avoid the problems I used to get as a lousy drywall amateur - keep working it, and the good part become bad, you introduce scrape gouges in what was smooth, you pick up crud bits.
Once the first coat is dry, you can solve any issues with incomplete feathering, etc., with another thin coat. A quick pass (ONE OR TWO ONLY) beforehand with a sandpaper or damp sponge takes off most of any problem bumps and ridges.
