I have earlier explained what Shoulder-In is and here are some more pointers to help you develop it correctly.
The most common mistake is to allow the outside shoulder of the horse to fall out.
Firstly you need to ride a bit more forward in a straight line and then make half halts on the outside rein to improve the balance, and so that you can feel the horse “sit up” on the outside rein.Hold onto the outside contact in a way you are almost pushing the horse upward to the inside track. Keep your inside hip forward, close the inside leg and ask for him to relax the body muscles and bend around it.
It is a matter of, with the correct impulsion, that you balance the amount of pressure needed from each of the aids.