Imagine your standing on the edge of a plane ready to jump off. As you look out before you you get a little queasy and wondered if this really was a good idea. In order to stay safe you'll need a parachute but why do you need a parachute? First you start out with equal force between your normal and your gravitational force equal as you sit on the edge of the plane. Then the moment you move your foot in order to jump out of the plane the gravitational force pulls you back towards the ground. After a while the air resistance will reach the same force as the gravitational force is at. This causes you to stop accelerating downward but if it stops with a velocity of about 2000 meters per second you are still falling at a constant pace and you don't want to hit the ground with a velocity about 125 miles per hour. At this point you would pull the string to release your parachute this increases the air resistance and slows your speed down to a speed that you would live at if you hit the ground at it.
The parachute adds to the air resistance because of its surface area and material it is made of. For an ideal parachute you would want one with a large amount of surface area, so the wind can get caught up underneath it and increase friction, and you would want it made out of the lightest material possible, so it doesn't add a significant amount to the gravitational force pulling you downward. If you had a heavy fabric as a parachute you would then increase both the air resistance slowing you down and the gravitational force pulling you faster towards the ground.
If you were to make your own parachute more precise details would have to be considered like the shape and balance of the canopy so you could increase the amount of air molecules that get trapped under it and the friction it will cause. You also might want to consider a more round shape because it is far easier to balance a round shape than that of a triangle. Canopy vents would be another consideration so you can limit how the air molecules will escape the parachute and this would also effect your balance. keeping you in the air longer and at a slower velocity downward so you can be safe when you reach the ground.