Firstly, find out what rug size your horse actually takes. A rug too big or too small will rub and cause issues. Then, find out what rug SHAPE best fits your horse. Something with a flatter topline? Curvy topline? Cut for a laid back shoulder or a straight one? My boy has a rug that I adore for him because it's actually cut for his upright shoulder, so it sits really well on him and doesn't rub.
Once you've done all that, start with a basic waterproof shell. Canvas is great because it breathes, but it also weighs a lot, especially when wet. Synthetic is light but [and I don't care what the rug companies have to say about it, I haven't found any claims to the contrary to be actually TRUE] it doesn't breathe. My 2yo TB is in a canvas that's only very lightly lined with a woven wool layer and she does well in that - mind you we are coming into summer now so soon both of mine will just be in cottons.
Depending on your horse you might find that just a shell is enough - keeps them dry and cuts wind chill factor - or you might need something with fill. Start with a light fill and work your way up. You'll find that even if your horse needs a heavy rug in winter, it's nice to have lighter ones as well so that you can transition her in and out of rugs without giving her the shock of her life! A shell is also brilliant for those spring and fall days where it's too warm to put something heavy on, but it's raining and it will be cold at night, so you HAVE to put something waterproof on so you're not rugging a wet horse in the evening.
I've been rugging every winter for the past 4 years solid, out of total necessity because some of the horses we have had have been the type that you can't keep weight on if they're cold no matter how much you feed them. In fact with my current gelding it's a case of better slightly too warm than slightly too cold, because he loses a heap of weight if he's stressy and for some reason the cold really freaks him out. With the TB filly, she doesn't need heavies [or rugs for that matter] full stop - I'm just rugging to try to keep her coat nice, because she's chestnut, and I hate the colour chestnuts go when they bleach out.
With the leg straps, you do up one leg strap, then slip the other between that and the leg it's around, clipping it up so that the straps are crossed. That way if the rug slips it's not going to cut off circulation or cause sores, and it's not going to slip all the way around. Make sure the opening of the clip is towards the horse's body because some horses can figure out how to undo them the other way round.
Belly straps/crossover surcingles should be snug enough that it's physically impossible to get a hoof caught in them, and the chest straps should be done up so that there's some room for the shoulder to move but not too loose or the rug will slip back and cause rubs [and possibly sores].
Rugs with shoulder gussets are best, I have found. Rugs without rub like you wouldn't believe. I'm not a fan of the weatherbeeta forward gusseted design although my boss has a ton of rugs with said feature and none of her horses have shoulder rubs so it can't be that bad. I'm just not into it.
Rump and shoulder darts are excellent when your horse is just "that" shape, but if they're in the wrong place or are the wrong size for your horse, then they cause problems.
Mostly, above all, make sure your rugs FIT, aren't too heavy or too light, and don't leak! It's worth paying for quality, too, because usually a quality rug will last longer than cheap rubbish - although I have a rug that's seen 5 or 6 winters now that cost $60 [very very cheap for a waterproof in Aus] and it's still going strong, not faded. Over the years my horses have wrecked 2 straps and the neck part so I converted it into a standard neck [it was a combo] and then retired it to the spare pile, but it's definitely a tough old thing.
A CLEAN SLATE FOR THE FUTURE