The only knot
Knots seem to be an eternal problem to anglers. Even experienced anglers have trouble remembering them, choosing amongst them and getting them right. Here's a cure: learn and use this one, and you'll be fine!
This is definitely going to get me in trouble! But I guarantee you that if you learn this knot you will be able to get very far in almost all kinds of flyfishing. Use this for your daily knotting and use loops for the rest (backing to fly line, fly line to leader), and you will be able to make it through many, many fishing days.
And I'm not talking about some new, space age, transformable, the-knot-to-end-all-knots knot. I'm talking about a true and simple classic.
Learn the surgeon's knot!
It's easy, versatile and useful for tying tippets to leaders, tying tags on tippets and tying flies on your line, which is what you will be doing a far majority of the time you tie knots while you're fishing... unless your fishing is very exotic and different, in which case you hopefully already know your Blood knots, Albrights, Bimini Twists or how to attach titanium wire traces and whatnot.
I'm not talking about some new, space age, transformable, the-knot-to-end-all-knots knot.
I can already hear the uproar:
"But it's not straight!"
"It's a loop knot and you must use a tight knot!"
"It's not good for different thicknesses of line!"
I know, I know, I know!
Of course there will be better knots for many applications, and the surgeon's knot won't cut it for tarpon leaders, for creating your own straight, smooth, tapered leaders, for joining leader and fly line and numerous other applications, but it will do fine in 90% of the cases if you fish for bass, rainbows, brown trout, salmon, seatrout and other "plain" fish, which don't require special leaders or complex rigs.
The knot essentially consists of a double stretch of line passed through a loop formed by the line itself. It can be tied in the blind, is very durable and even compact. It does have a tendency to become crooked and isn't as smooth and nice as some other knots, but the ease and durability more than compensates for this. It can be used for many purposes, and I will describe three very common uses:
Adding a tippet to the leader
When you fly fish, you constantly add tippet. Next to tying on a new fly, it's most likely the instance where you most often will need to tie knots.
The surgeon's knot is perfect for this. It's easy, it's compact, it's strong. And it shows the basic technique for tying the knot.
And again: yes, I know there are tonnes of other neater, straighter, more beautiful knots to use, but this one is simple, efficient and versatile.
This is how it's done in text:
Cut a piece of tippet according to your needs. A couple of feet or three or a little less than a meter is usually a good choice, but of course it depends on the circumstances. The thickness depends on your leader and the fishing you do, but in general it should match the thickness of the tip of the leader or be slightly thinner.
Take your leader and grab about 5-10 inches or some 10-20 centimeters of its terminal end. The shorter this piece, the less the waste of leader material.
Line up your newly cut tippet parallel to it, overlapping the same length.
Fold both lines to form a loop on the double stretch.
Pass the end of the leader and the business end of the tippet through the loop.
Repeat to get two passes.
Tighten the knot slowly and make sure the lines stay neat and parallel.
Moisten the knot and draw tight.
Cut the tags and you're done!
Tying on a fly
The Surgeon's knot will form a loop knot when used to tie on a fly. Some people might not want their fly working independently of the tippet, but in most cases this is actually an advantage. I use loops for almost all my fishing: dries, nymphs and wet flies, even when fishing for salmon. Some might find this sacrilegious. I don't care. It works.
There is one disadvantage of using a loop: on thinner lines the connection will be weaker than when using some tight knots. But regular fly changes and inspection of the loop will ensure you that things are OK and keep break-offs to a minimum.
The knot is tied like this:
Pass the tippet through the eye of the fly and double it for about 5-10 inches or 10-20 centimeters, depending on your dexterity and the size of the fly and thickness of the tippet. The less you double, the less tippet you waste.
Form a loop on the doubled line. The fly has to be able to pass through this loop twice, which doesn't necessarily mean that the loop has to be huge.
Pass the fly through the loop twice. Sometimes passing it hook point first is the easiest. Materials flex, the hook doesn't.
Tighten the knot while keeping the loop as small as possible. One easy way of doing this is slipping the double loop over the eye of the hook while tightening, and letting it slip off just as it gets small enough. This will give the smallest possible loop, usually only a bit larger than the eye on the fly.
Moisten the knot as you tighten it fully, and cut the tag.
Adding a dropper tag to your leader or tippet
If you want to fish more than one fly - hopper/dropper style or a point fly and a dropper in the UK terminology - you will need a tag for the top fly AKA the dropper. This is also very easy to do with a surgeon's knot.
But you have to make a decision before commencing: upwards or downwards dropper?
Downwards gives you a smoother rig, but with a slightly greater risk of tangles. On this rig the top fly (dropper) is usually tied to the main line, and the lower (point) fly is tied to the tag, which elongates the tippet.
Upwards makes the dropper stand out from the main line, but might be slightly weaker (I never experienced it as weaker). When using this method the top fly is tied to the tag while the end fly is tied to the regular tippet, which continues through the knot.
Downwards tag: tie on your tippet, optionally shorter than usual. You will have a longer rig once the tag is tied on. Cut a piece of line, same or thinner than your regular tippet. The length is decided by what distance you want between your flies. Lay it along the standing line so that the regular tippet forms a short tag and the added tippet forms a continuation of the normal tippet. Form a loop and pass the terminal ends through twice. Moisten and tighten.
Tie your top fly (a dry, a hopper, a large fly) to the short tag formed by the regular tippet and your lower fly (a nymph or wet fly) to the long tag formed by the added line.
Upwards tag: With this rig the length will be as the regular tippet, so tie this as normal or even a bit longer to have something to work with. Cut a piece of line, same or thinner than your regular tippet. The length is decided by how long you want your tag. Lay it along the standing line so that the regular tippet lies as usual and the new line forms a tag upwards towards the fly line. Form a loop and pass the terminal ends through twice. Moisten and tighten.
Tie your top fly to to newly added tag and your lower fly to the regular tippet.
Quickly adding a second fly
Using a hybrid of these methods you can also quickly add a tag to a rig that already has a fly mounted. Add a second fly over or under the fly you are fishing already.
Make the tag short and upwards for a fly above an fly and long and downwards for a fly below an existing fly.
New fly above the existing: Cut a short piece of line (½-1' or 15-30 centimeters), similar to or thinner than the tippet. Grab the tippet or the leader where you want the new top fly to attach. Choose to point the tag upwards or downwards according to taste and form a loop on both lines. Pass the fly and the two lines twice through the loop, moisten and tighten, and you will have a tag to use for a new top fly.
New fly below the existing: Cut a longer piece (3' or 1 meter por even longer) of line similar to your tippet. Lay it along the tippet ½' or 15 centimeters above the fly with the long tag downwards and form a loop on the parallel lines. Pass the fly and the long tag through the loop twice and attach the new bottom fly to the new, long tag.
Now learn some more!
OK, I admit it: I would personally never be able to do with one knot!
Even though the surgeon's knot is really, really useful and I get by with it up towards 80 or 90% of the time I need a knot, there are limits, and learning a barrel or blood knot, a Palomar knot, a Trilene knot, the perfection loop and even the nail knots will bring you way farther than the humble surgeon's knot can.
I'm often surprised how conservative fly anglers are
I'm often surprised how conservative fly anglers are, and many never learn more than the first few knots they saw. Learn more knots and get to know their advantages and disadvantages. Practice a bit and make sure you can tie the most useful knots that enable you to set up a complete fly rig in the field with no tools or chemicals. That will save many a trip for yourself and your fellow anglers.
These articles also touch upon knots and/or fishing multiple flies