The Forward Slip is a slip usually used to lose altitude quickly. Forward Slips are never to be done with full flaps extended on the aircraft. Forward slips are demonstrated by choosing a road or runway with a direct headwind or tailwind. You will apply opposite rudder and aileron (e.g. left rudder and right aileron or visa versa) as needed to maintain a straight flight and ground path. You will be traveling in a straight line, even though your nose will be pointed to the left or right.
The Side Slip is most commonly used for crosswind landings. By its very nature, it will also cause you to lose altitude due to the loss of vertical lift over the wings. The Side Slip is demonstrated by choosing a road or runway with a direct crosswind. You will place your ailerons into the wind as much as is necessary to keep you lined up with your chosen road or runway and not be blown off course due to the winds. You will then apply opposite rudder to line your nose up directly with the road. Your flight path will actually be slipping to the left or right-into the wind-while your ground path and your airplane will remain lined up perfectly straight.