Find a line that travels across the entire slope that you want to measure.
Measure the vertical height of the slope. You should measure the height of the slope at its highest point. For example, if you're measuring the slope of a skateboard ramp, then you should measure it from the ground to the highest point of the ramp.
Measure the horizontal length of the slope. For example, if you're measuring the slope of a skateboard ramp, then you should measure the ramp's base, from the frontmost tip to the very back.
Sketch an image of a right triangle, some of the dimensions of which you have already measured. You should have the sloped line as your hypotenuse and the vertical height and horizontal length as your other two sides. This sketch is optional, but it helps you to visually depict the geometrical aspects of the slope.
Divide the height of the slope by the length of the slope. This is what teachers often refer to as "rise over run," which means the rise of a slope divided by the run or the horizontal distance. The resulting number is your slope gradient, which can be expressed as a fraction or decimal.