A "necessary" but not "sufficient" condition for learning quarter body movement is to be able to "fold" the body at the spine. Click the link for the video.
In the beginning try to move both halves of the body, then you can try form movements such as "single whip", "parting hands" etc. where only one half is moved. As you do these , do not tip your head but you can piecemeal look down at your shoulder joints to discern any extraneous external movement. You can watch the shoulder joints in a mirror, video, via a friend's critique, etc.
Make sure to relax the back both upper and lower to facilitate the practice. Relaxing includes "SONG KUA" OR AS IT IS CALLED "STRETCHING DOWN THE TAILBONE" and we have discussed this in earlier posts. THIS WAY THE FOLDING CAN EXTEND THE LENGTH OF THE SPINE. STRETCHING THE TAILBONE DOWN RELAXES THE ENTIRE BACK.
Teaching "not to move" is more difficult than teaching "how to move"..."How does one teach/learn upper body movements? MINIMIZE EXTRANEOUS EXTERNAL MOVEMENTS first, such as a lift of the shoulder, excess motion at the shoulder joint, stretching the arm out, pulling the elbow in, and swinging the arm without purpose or hand flourishes. Only when external movements are minimized can the internal movements be free to fluorish. That is one of the purposes of square form, freezing the arm from external movements during certain form movements"
These are examples of many "folding" movements of which its ability of doing is essential for any future quarter body movements. Don't fold at the shoulder but fold along the spine to the tailbone. One can develop a knack for moving from the core. Find a neural pathway in the body core abdomen/back which can make the internal move you intend. Get through this first "stage" then work toward a second "stage" where arms are integrated with those internal movements.