to start, take your front and back pattern pieces and line them up at the shoulder's seam allowance. tape it together then place tracing paper on top. trace the armhole between the ease points and mark the shoulder seam. make sure to label which side is the front and which side is the back, as they are not symmetrical. then just draw a curved line up to about 1" from the neck seam.
determine how many layers of fleece/batting you need. for me, three layers gave me 3/8" which is a hair thicker than the 1/4" called for in the pattern. since fleece is more squishable than batting, it will work out perfectly.
cut each layer to be about 3/4" smaller than the one before it. make sure your shoulder seam is marked on each piece so you can line them up easily. now, curving them in your hand, make a running stitch along the straight edge to keep it all together. don't bother with knotting your threads, just take a back stitch at either end.
once you have done this, make radial stitches throughout the shoulder pad starting from the center and shaping it as you go. once you're done, your pad should hold a curve on it's own.
now cut two pieces of muslin and pin it together around the shoulder pad and trim the excess from the bottom layer so it is even. then serge or zig-zag around the rounded edge (careful to maintain the curved shape) to close it up.
on the straight edge, just whip stitch the muslin to the next layer of fleece/batting to keep it all together without squishing it down.
and there you have it... custom shoulder pads!