The fine folks over at IGN have done us all a favour. They have single handedly located, performed and succeeded all 50 stunt jumps on the GTA 5 map.
Stunt Jumps are hidden ramps propelling vehicles into slow-motion jumps. Upon completing all 50 Stunt Jumps you are awarded with the “Show Off” achievement. It’s best to do them while playing as Franklin because of his special ability to slow down time while driving. The best vehicle for these Stunts is the Pegassi Bati 801 motorbike. In addition, you must always land in a certain area, or sometimes a very specific spot to complete the stunt jump; if you drive too fast or too slow, you might miss the target and the stunt will fail.
IGN have also gone the extra step further and created an interactive map that marks each stunt jump location and in conjunction with this they have also compiled a written list that describes the location of each Stunt Jump.