SLIPS by Scott Gibbons
A slip is a useful maneuver that can be used to adjust glide path to a steeper angle during the landing approach. A slip is executed by cross-controlling the ailerons and rudder. For instance, if the airplane was high on the approach, a slip could be initiated by adding left rudder and right aileron (or right rudder and left aileron). This will result in a higher sink rate on the approach due to increased drag. Once the appropriate glide angle (or sink rate) is achieved, the rudder can be returned to neutral along with corresponding reduction in aileron.
As a technique, I will add left rudder on the approach if I think I am high. The aircraft would naturally then turn to the left if no other control inputs were given. To keep the airplane aligned with the runway, I then add in right aileron as needed. You can think about it as holding a constant amount of rudder, and then using aileron to align the airplane with the runway. The benefit of this technique is that the aileron control is normally used to fly the airplane on approach, so there is not much difference between the slip and a normal approach.
A slight increase in stall speed will occur due to the cross-control condition. Due to this, the approach speed should be increased during the slip by an appropriate amount depending on the model attributes. Increased approach speed also increases parasite drag and will further increase the rate of descent.
Note that the airplane is not likely to spin during a slip. A spin would be more likely to occur during a skid, in which excess rudder is applied in the direction of a turn.