I’m not a leader building expert, but I’ll try and walk you thru it, based on my understanding, but I hope others will chime in if I’m way off base here.
There are basically 3 types of leaders:
Furled- made of individual strands of thread, mono, or fluoro that are “furled” or twisted upon themselves into what looks like a rope and then regular tippet is added. Many people swear by them, some swear at them. But if you’re interested in making leaders these are definitely worth checking out, and a lot of board members use and make them. Joni is the goddess of furled leaders.
Knotless- extruded mono that tapers in thickness from thick to thin. The advantage is that they don’t pick up moss or algae on the knots like Jakeway said, and are easy to use. Just buy one the desired length, usually 7 ½,’ 9’ or 12’ tapered down to the desired thickness 0X-8X, and open the pack tie it on and add a section of the desired tippet. The knotless leaders can be expensive however.
Knotted- homemade leaders let you fine tune the leader for different applications. It seems like you might be able to save money tying your own given the price of premade knotless leaders, but the catch is that you’ll need a lot of different spools for fishing small flies because of the number of step downs you’ll need to do between sections of different diameters.
Leaving aside furled leaders there are some basic guidelines in construction for both knotless and knotted leaders.
Typically they start with a butt section connected to the fly line approx 2/3 the diameter of the fly line to ensure smooth energy transfer and gradually get thinner from there.
Because the knotless leader is extruded this is more or less a continuous process.
With knotted leaders the difference is more obvious, and this is where things start to get complicated. To ensure good knot strength and even transfer of energy, the difference between the sections of a knotted leader need to be within .001” or .002” of each other. More about this in a bit.
As Glassroddr said, the 60-20-20 refers to the butt, mid section and tippet lengths as a percentage of the overall length of the leader.
OK sounds simple. Let’s try to build one, say a 9’ leader tapered to 5X to use on a 5 or 6 weight rod. I haven't actually built this or fished with it, but I think it might be a good look at the process of building one from scratch. Hopefully I'm not all full of sauce here. Please jump in here if anyone can explain it better or point out where i might be off base.
The overall length of a 9’ leader is 108”, so we know:
butt section is 108” x .60 = 65” (64.8”)
mid section is 108” x .20 = 20” (21.6”)
tippet section is 108” x .20 = 20” (21.6”)
We’ve lost a few inches here because of rounding, but let’s press on.
We also know the section of butt attached to the fly line needs to be roughly 2/3 the diameter of the fly line. For a 5 or 6 weight, that would be .021”. (For 1-4 weights you could use .019).
And we know the tippet we want is 5x. To get the diameter for tippets 0x to 8X take 11 and subtract the “X” number to get .000”. So for a 5X tippet, doing the math, 11 - 5 (the X) = .006” thick.
Hmmmmm. We know the thickness of the butt section attached to the fly line is .021” diameter, and the tippet is .006” diameter. We want to get from .021” to .006” so we need something in the middle. We also know that we want a gradual transition from thick to thin within .001 or .002” between sections.
So, if we skip by .002” we’d need sections of:
.021 (to fly line)
.06 (5x tippet)
or 9 different sections altogether.
At this point it looks like we’re screwed. But some general guidelines for building a leader are:
Butt section total length 60%, with individual sections also with proportional lengths within the butt section, with thickest being longest
Mid section- 20% of the overall leader length, with individual sections of equal length, but you can’t just put a ton of short sections in the middle because you’ll just have knots, which will be thick, effectively messing up the whole concept of going from thick to thin diameters.. Lets’s try and keep each section of the middle to a minimum of 6”. In this case the mid section is about 20”.
Tippet, this is the easy one, just tippet here and we know it should be 20% of the overall leader length. So in this case we’ll have about 20” of 5X at .006
Since we have a lot of different sections here, and want to keep things a little manageable, let’s not kill ourselves and try to get this exact, but it also makes sense to try and work it out on paper first.
Let’s make the tippet 20” of 006” (5x)
We’ll make the mid section 24” overall with 4 different diameter sections each 6” long just to be able to get whole numbers. Working backwards from the tippet, this wlll be:
6” of .007 to attach to the 5X tippet which is .006
6” of .009
6” of .011
6” of .013
For the butt section, we know the overall length for a 9’ leader is 108” x .60 = is 65 “ and we have to taper down from .021 to .015 to get to within .002 of the thickest portion of the mid section, and also know we want the sections in the butt to be within .002 of each other. So we’ll need 4 sections in the butt:
Since the butt section is supposed to be proportional, with the thickest section the longest, and we have 4 sections, we could build it with something like a 4-3-2-1 ratio, or:
40% of 65” = 26” of .021
30% of 65” = 20” of .019
20% of 65” = 13” of .017
10% of 65” = 7” of .015
So, for a 9’ leader for a 5 weight rod tapered to 5x we’d have
26” of .021 butt
20” of .019 butt
13” of .017 butt
7” of .015 butt
6” of .013 mid section
6” of .011 (0X) mid section
6” of .009 (2X) mid section
6” of .007 (4X) mid section
20” of .006 (5x) tippet
for a total length of 110”, close enough.
The next step is to actually test it with a fly to see if that dog will hunt, and more than likely tinkering with the butt section. It’s a lot of work to go thru, but fortunately you don’t have to reinvent the wheel. There are a bunch of different leader charts out there, some follow the 60-20-20 type of construction, some don’t:
and if you have Excel:
Hope this helps.