A side slip is a manoeuvre in which the aircraft is placed in a banked attitude, and the natural tendency for the a/c to yaw is countered with opposite rudder. This just looses height rapidly.
Entering and recovering from a forward slip is similar, but more skill is required as there is an intended descent path to follow. This is done by using sufficient rudder input to track towards a feature such as an initail aiming point in a field.