Get a small clean drain plunger. Use the simple style of plunger without the flange in the bottom.
Clean the body of the car so that it is free from grease and dirt.
Put a bit of soapy water on the rim of the plunger, not too much. This will help the plunger create suction against the clean car body.
Attach it to the car body over the dent. Create an adequate amount of suction.
Slowly pull the plunger until the dent is gone.