If you have narrow shoulders, you need to lift the sleeve in on the shoulder and adjust the sleeve a bit.
You Are Working With The Front Piece, The Back Piece And The Sleeve.
1. Mark the shoulder width:
Mark on your muslin where you want the sleeve to sit and draw it on to the fabric.
Transfer your marking to the pattern on both the front piece and the back piece.
2. Raise the armscye:
Top balance the sleeve so it does not pull, you must raise the sleeve cap. The amount you need to raise it is very individual. If you have slim upper arms, you will not need to lift it as much as you do if you have larger upper arms. Raise the sleeve cap by an amount between 0.5 cm and the amount you took off the shoulder seam.