Goal: Patch holes in tarp to make waterproof.

Which of the following methods is more reasonable for accomplishing this goal?
You can use duct tape to patch holes in the tarp to make waterproof. Start at one end of tarp and hold up to sun looking from the bottom any holes will show visible light where they are located mark with a marker , use one darker or lighter than the tarp color. When finish patch holes on a flat surface appling the duct tape and smoothing out with your hand.