When you fly the slip you do not adjust the speed by reference to the ASI!
You fly the slip by flying the attitude.
If you chase the ASI you will invariably go too fast which makes it difficult to stay in the correct attitude (in pitch, yaw and roll) and you will be way too fast when you come out the slip.
This will then mean you are in a high energy situation close to the ground and a crap landing will follow.
To master the slip the pre-requisite is that you have an experienced instructor or tutor who knows what they are talking about explain the theory and show you how it is done.
Once you have the basics under control you go up and practice slips, left and right, high up along line features until the cows come home and can do them without even having to think about what the inputs are which are required.
You then go and practice with your mentor the slip before landing and progress from there onwards.
FD