Slim upper arm
Pin away extra fabric.
Mark the pin placement and draw the lines on the muslin using a ruler. This makes it easier to see how much you need to alter the pattern.
NB! Your Pattern Pieces Might Not Look Like Mine Exactly. Follow Your Markings.
You are working with the front piece, and sleeve
Transfer all your markings from the muslin to your pattern. Do this by measuring on the muslin and then mark the pattern piece in the same way. The red lines are the new lines.
2. Cut the red lines.
When you make a sleeve tighter on the upper arm, it is often because you have a full bust and need to make the armscye smaller.
Make the cutline on the sleeve curve; then it is easy to assemble in the later step.
3. You will not work with the small piece, which is the excess fabric
4. Re-join the pieces of the sleeve.
They have to fit at the bottom
Make a new sleeve head curve; it has to cut in the middle of the gab that occurred when you re-joined the two pieces.
5. Cut the excess paper of the sleeve head away and tape some extra paper under the arm, and you are done