Key points... (1) the purpose of a leader can be manyfold however the main purpose of tapering is to allow a smooth progression of energy from the flyline to the fly, both allowing it to progress but also dissipating it. If you have no taper and only tippet then the flyline will kick - which is what I often see that when looking at other people's setups. However even more important IMO is that you need a consistent cast, and then, and only then, can you experiment with leader design. So learn to throw! Once you've done that you'll find that the butt section of your leader should be of a very similar diameter to that of your flyline. I think Bruce Richards recommends a diameter of at least 2/3rds that of the flyline - but you can make it 100% and taper from there.
(2) When using more than one fly, the spacing and size of the flies are critical. Think bushy and bigger, or smaller, at the top fly (we call that the Bob Fly in the UK, top dropper elsewhere), middle fly should be that, and the point fly can either be heavier or indeed smaller. Mostly I make the point fly the heaviest. The key point here is to make sure that the distance between the point fly and the middle fly is at least 1.5 times the distance between the middle fly and the Bob Fly. Preferably more.
(3) Many years ago I wrote an article on Sexyloops saying that the Bob Fly was named after Robert Dibble who invented the method of skating (or dibbling) the top dropper in the surface film. Not very funny in itself - you had to be there - however one year later another Robert Dibble wrote to me saying that he was from the West Country and was he related?
For trout fishing in NZ, Europe, Tasmania, North America, anywhere there is bloody trout, I use a 9ft tapered leader. I like RIO 9ft tapered leaders to 10lbs point. They have a good stiffness and there is something about the tapering that is excellent. Varivas also make excellent tapered leaders, but I can't remember which one I liked - there are two tapers. I've tried SA leaders but find them too limp. Maybe it's just the ones I've tried because I'm sure that they must be making good ones too - after all Bruce Richards designs this stuff.
Now in the case of trout I'm using either 4 or 6wt Hot Torpedo rods (because this is the best rod in the world of course) and I connect the leader to the flyline using a 2.5 turns Needle Knot. This is the absolute best knot because it will easily slip through the rod guides, which is very important because I typically use long leaders when trout fishing. So if your trout flyline has a crappy welded loop then cut the damn thing off.
For trouting, I then add a small Sheepgirl Ring. If you've been around SL for a while then you'll know what this is - it's a little silver ring stolen from your girlfriend's necklace, preferably a sheep herder, and it helps keep the integrity of the leader for up to a week of intensive fly fishing. And after this it all happens...
For three flies stillwater fishing, my leader is typically 22ft long. 9ft tapered leader, 2 feet to Bob Fly, 3 more feet to middle, 7 or 8 more feet to point fly. It's approximate but the key is to have the point fly to middle at least 1.5 times the distance as from middle to bob, otherwise you'll get a lot of tangles.
There is one exception of course, and that's when fishing a three fly rig in NZ; 9ft tapered leader to 10lbs point. Sheepgirl Ring. 2 feet to top dropper. 6 plus feet to "Point Fly", 12-18 inches to trailing nymph.
If I only fish one fly, maybe a dry fly or a streamer, then I'll typically shorten my leader. This might be Key Point 5 (because there is no number 4 on Sexyloops) - as a good rule of thumb your leader should be at least 1.5 times the length of the rod. Less than that causes many problems in my experience. Either tailing loops on turnover, or else the fly turns over too hard. So I'll use my 9ft tapered leader and then add 4 or 5 feet of tippet. When fishing a very heavy fly I might shorten this... and so...
Here's what I fish for Snakehead on an 8WT rod: 4-5ft Butt section 70lbs mono, welded loop in the flyline, perfection loop in the mono. 6 feet 40lbs mono, Bimini Twist into a Slim Beauty, six inches 30lbs Tyger Wire with a Slim Beauty. Lefty Knot. This leader breaks at around 27lbs in tug of war.
For Gourami, 4WT HT, a very cool braided leader from Graeme in Perth, WA, that is braid with varnish and oil mix to add weight - 9ft long, followed by 50lbs braid, Bimini Twist loop (learn this knot - it's the most amazing knot we have), either straight to the fly or with a finer Spectra tippet of approx 30lbs. Penny knot/Eugene Bend to the fly - the same knot, just different names.
Anyway that's my starting point when designing leaders. It's quite easy to add more tippet in many cases, particularly if you want slack line. In fact I'd rather a use a leader that wants to collapse and check haul it to ensure turnover, that have a leader that always straightens with force. Apart from with Spectra braid which is just a pain in the arse to deal with, but very important here in the jungle.
So key points: 1) the leader should as a general rule be 1.5 times the length of the rod or more. With heavy flies you can shorten up, to around 10ft. 2) You must have a butt section of a similar diameter to that of the fly line. Many casting problems originate here. Of course you can intentionally collapse the cast at this point by inserting a smaller diameter. 3) Robert Dibble invented the Bob Fly. 5) You need to buy a Hot Torpedo fly rod when you are ready for the best fly rod in the world. 6) Learn the "Check Haul", the Bimini Twist and the Slim Beauty.