This is a Strangers and Freaks side-mission for Franklin.

Start by following the dog you see. Franklin will have a conversation with the dog on the way.

Eventually, you will find a man with a parachute stuck in a tree, approach him to help him down.

The man, Dom (The Dominator), will pursuade Franklin to try skydiving. Franklin will reluctantly accept the offer. Follow the man and eventually his helicopter will arrive. Climb into the left side.

Once in the helicopter, the group will fly to a drop-zone, you may 'skip the journey' to speed this up. When you arrive at the mountain, you will have to jump from the helicopter.

When falling, aim for the red/orange target. After landing, hop on one of the two bikes that are sitting at the top of the trail. Race Dom down the path. After the race, you will receive Dom as a contact and have the Parachute Jumping activity will become unlocked.