Leaders, otherwise known as casts, should be tapered. This is to enable a smooth transition of power from the fly line through to the fly. Go for as stiff and as thick a monofilament taper as you can knot to the flyline. Leaders should be one and a half times the length of the rod. However you may need to change as circumstances dictate. For example short leaders for the booby, and longer ones for teams of dries. I love copolymer material. It is thinner and less forgiving to poor knots and kinks, but the finer diameter gives me far more takes. For knots use three turn water knots and half blood knots.
Various recommended leader set-ups are shown. For teams of flies on sinking lines I use the shorter leader system.
For sinking lines of a sink rate quicker than an intermediate, I replace the tapered leader with a straight length of 15 lb mono (it's cheaper, and the trout won't know because they're too deep to see the flies land).