Solve your parabola's equation y = (x - h)^2 + k so that the x term (x - h) equals zero. If, for example, your parabola's equation is y = (x - 2)^2 + 5, this would make x = 2, since 2 - 2 = 0.
Draw a vertical line on your coordinate plane at x = 2. Make a dot along the x-axis at this value, then use a ruler (if necessary) to make the line.
Plot your parabola on the coordinate plane for various values of x. For the example equation y = (x - 2)^2 + 5, plug in the following values for x = 0, 1, 2, 3 and 4. These will yield y = 9, 6, 5, 6 and 9. When you plot the graph on your coordinate plane, you will notice that the parabola falls until x = 2 and begins rising again after it; x = 2 is this parabola's line of symmetry. For parabolas in the form y = (x - h)^2 + k, x = h is always the line of symmetry.