Tie a loose figure eight, but do not leave a long bitter end.
Trace the bitter end of the other line back through this figure eight so the bitter ends are on opposite sides of the knot.
Grasp all lines on each side of the knot and draw them up tight.
To join two lines:
To create a tie point in the middle of a line:
Take a large bight where you want the tie point.
Tie a figure eight with that bight (you are basically just doubling up your line and tying a figure eight).
Climber's variation of the figure eight for a tie point:
Orient the line up and down rather than side to side.
Form a large sideways "S" with the line
Using the upper loop of the 's' go around the standing part
Tuck the bight through the lower loop
Draw it tight and the knot is complete.
As you can see, the figure eight and its variations are very versatile with a variety of uses whether extending your rope, putting an eye in a line or as a stopper on the end. These variations are both secure and will not bind, well worth the effort to learn.