To attach a leader to your mainline for trolling, tie a double uni-knot or a blood knot. These knots are strong and will prevent your leader from breaking when you hook a fish. Make sure your leader is at least 6 feet long and use a heavier line weight than your mainline to prevent the fish from breaking off. Attach your lure or bait to the end of the leader using a suitable knot for the type of lure you're using.