Bear in mind: WHAT I'M ILLUSTRATING IS BODY SPECIFIC FOR MY FIGURE. This may not work on someone else's figure, so it would be prudent to check out my slide show to see if my figure is similar to yours. I'm working off of a 42-year old body with forward shoulders and broad upper back. Long arms, long legs, short waist, rectangular torso with not a whole lot of waist definition, 5' 7" in height, somewhat slender, hollow chest, full bust (propped up most of the time with a Wonder Bra LOL), and narrow shoulders.
So, knowing that, let's proceed...
TUTORIAL: Altering a Sleeve for Forward Shoulder
Step 1 - Alter the bodice.
Let's assume it's a 1/2" forward shoulder alteration. Remove 1/2" from the front as shown and add 1/2" in the back as shown. Another option to consider is moving the entire shoulder seam 1/2" forward. In the pic below I'm tapering to 0 at the neckline rather than moving the entire shoulder seam forward. Whatever works best.
Step 2 - Examine the sleeve pattern. Most sleeve patterns I see are like the one below and symetrical. If this is the case, the sleeve will need to be altered for a better fit since this draft does not mirror a typical figure. Draw a vertical line from the center of the sleeve cap at the top to the middle of the hem at the bottom. Draw a horizontal line perpendicular to the vertical line across the sleeve right above the notches. Also draw in a hash mark on the horizontal line a 1/2" away from the vertical line toward the front of the sleeve as shown.
Step 3 - Cut the pattern on the horizontal line. Move the upper pattern forward a 1/2" so the vertical line on the top piece is aligned with the hash mark.
Step 4 - True the cutting lines. Normally I don't true my lines this way except for sleeves. Flatten/true the back as shown, bubble and true the front as shown. The sleeve is now asymetrical reflecting a true body shape.
Normally, this would be the end of it, but not for me...what else is new. I measure the SEAMLINE of the front and back of the sleeve separately as shown below. Subtract the seam allowances from your measurements.
Then I measure the front and back armholes separately on the bodice pieces. Subtract the seam allowances from these measurements.
Now I make comparisons of the front sleeve seamline and front bodice armhole seamline and the back sleeve seamline and back bodice armhole seamline as shown on my "math" sheet below.
If there is more than a 1/2" difference, I adjust the sleeve. Usually there is no problem on the back, but there is too much ease on the front to the point where it would look gathered. Therefore, using the above example, the front sleeve has 3/4" excess of fabric over that 1/2" of preferred ease. So I reduce the ease in the front of the sleeve as shown below. Draw a line (shown in green) 3/4" above the horizontal line.
Slash the green line and overlap it to the blue line to reduce the ease as shown below.
True the cutting line as shown below:
And that completes the alteration. I do this for almost every sleeve. I hope this information is helpful to one of my body twins out there.