Ask yourself the question: "What does the line represent?" Most often, a line that you would be asked to draw 15-degree intervals on is the equator, which wraps around the Earth at the center.
Measure your line with a ruler.
Divide the line into 24 equal segments (because 24 * 15 degrees = 360 degrees) if your line represents the equator. For example, if your line (in Step 2) was 12 inches, then:
12 inches / 24 = 0.5 inches
Draw vertical lines at each interval using the right angled set square. Start at zero. For the 12-inch line used as an example, you would draw vertical lines at the 0-inch mark, the 0.5-inch mark, and every 0.5 inches until the 12-inch mark, where you would draw the final vertical line.