CubCAPTAIN

Make sure you are putting the wing down into the wind. Dipping your wing downwind results in a less effective slip and is not safe in a crosswind; when you transition from forward slip to side-slip, you want to have that wing already down into the wind.

A good simple way to break it down when you start out is:
1)The amount of rudder deflection in the forward slip controls the rate of descent (more rudder=greater sink) The second half of the rudder deflection usually has a lot more effect than the first half.
2)Aileron input controls drift from side to side (aka helps you keep it on the centerline)
This is obviously a simplified way to think about a forward slip, as the rudder and aileron do not operate independently in a slip (for example: if you are slipping left wing down and the airplane is drifting left, you can correct by reducing left aileron and adding right rudder), but its a good way to start to think about it.

Also, remember that most airspeed indicators are not accurate when the aircraft is in a forward slip. Keep the same pitch attitude in the slip as you had before starting it, and your airspeed should remain safe. A lot of people drop the nose considerably when slipping because they are trying to lose altitude. The increased airspeed will usually result in using as much runway as if you hadn;t slipped in the first place.

Hopefully that makes sense