The forward slip can be a useful skill to master since it allows for fine-adjustment of the airplane’s height above the runway on final approach, allowing you to stay on glide path while maintaining your target approach airspeed.
Slips are useful when you’re high and need to lose extra altitude in a hurry to make your touchdown spot. In these cases, it makes little sense to just lower the nose to get down to the runway since that will cause a corresponding increase in airspeed. The skill you need to hone is the ability to dissipate the extra altitude without increasing airspeed above that required to land.
But it’s important to understand that not all slips are created equal. In a forward slip, the amount of slip, and therefore the sink rate, is determined by the bank angle. In other words, the steeper the bank, the steeper the descent.
In most light airplanes, of course, the steepness of the slip is limited by the amount of rudder travel available. This the practical slip limit, and any additional bank will cause the airplane to turn even with full opposite rudder applied. (You can, of course, lower the nose in this scenario, which will increase your airspeed and make the rudder more effective.)
To perform a forward slip to landing, bring the power to idle and slow to normal approach speed. Pick a point far in front of you and make it your target. To set up the slip, bank into the wind. As soon as you see your target start moving in the windscreen, apply opposite rudder to keep you moving toward your target. For example, apply right aileron and left rudder, or vice versa, depending on the direction of the wind.
Make sure the nose of your airplane during the slip is at the proper attitude to maintain your approach airspeed. To recover from the slip, simultaneously release the rudder pedal and level the airplane with the ailerons.
And remember, like most things in flying, practice makes perfect. If you’re uncomfortable with the concept of the forward slip, ask your instructor to go with you so you can work on the basics.