iFixit

Start removing the rear panel on the side with the buttons.

Slide the dental pick completely into the square hole. You may need to wiggle the tool and apply a good amount of force to get it to go all the way.

Pry up the rear panel by pulling the dental pick counterclockwise in a quick motion. Apply force perpendicular to the plane of the display assembly. You may be afraid to go fast, but going slowly is more likely to bend the case.

Use a spudger to free the rear panel from the iPhone along the corner and top.

Repeat the same procedure on the other side of the iPhone. Slide the dental pick completely into the square hole. You may need to wiggle the tool and apply a good amount of force to get it to go all the way.

Pry up the rear panel by pulling the dental pick clockwise in a quick motion. Apply force perpendicular to the plane of the display assembly. You may be afraid to go fast, but going slowly is more likely to bend the case.

The rear panel is still attached to the iPhone by the headphone jack cable, so don't entirely remove the rear panel from the iPhone just yet.

Make sure the phone is off before disconnecting the headphone jack cable.

Use a spudger to disconnect the headphone jack cable from the logic board.