Building or Repairing Monofilament Leaders

Every fly fisherman needs to know how to build and repair leaders. More often than not there are distinct advantages to building your own leaders. You can fine tune the taper and length to suit conditions.

There are many knot alternatives but I'm going to focus on basic connections.
You can use a needle knot or a nail knot to make the connection between the butt section of the leader and the fly line. With small-diameter lines, and also with sinking lines, needle knots can be a challenge so I simply use nail knots.

My butt sections are essentially permanently installed with a perfection loop at the end, which facilitates ease and flexibility in changing leaders.
As an option, you can leave the butt section intact and tie it to the leader with a blood knot. When working with larger diameters, meaning.010 inches and heavier, I use the common blood knot. When joining smaller diameters, I'll use the surgeon's knot.
Take the line through the loop three times, and you've got a double surgeon's knot which you should use with fluorocarbon material. Not only are surgeon's knots strong (almost 100%) they're easier and quicker to tie, and they are more reliable when connecting dissimilar types of monofilament.

For a typical leader-to-tippet connection I use a surgeon's knot. For connecting very fine-diameter stuff, or if I'm in a hurry, and want to jump more than two-thousandths (.002), I add one more turn, and the surgeon's knot becomes a double.

If you want to really get into tying your own leaders I'd suggest a visit to one of the many internet sites that offer pretty explicit information on the subject. One of the best is the Global Fly Fisher which even offers fee software, LeaderCalc. It's a spreadsheet tool to calculate the length and diameter for each segment in a hand-tied leader. LeaderCalc contains 70+ of the most common and popular leader formulas ranging from delicate dry fly fishing to stout bass popper fishing.