I bought this great ruffled tank top from Old Navy (on sale yey!) but I knew the straps were way too long. The neckline bordered on vulgar and there was no way I could ever wear it outside. I couldn't even wear it in these photos so I put my shameless mannequin on the spot and figured out how high I needed to bring up the straps to create a bit of modesty.
I needed to take off 1.5" from the front and back (3" total) so I cut the shoulder seam off and then 1" off the front and back of the strap. I left 1/2" on each side to use as my seam allowance.
This is where it gets tricky, and the black fabric doesn't help. Scroll all the way to the bottom for an illustrated tutorial of the lapped seam. Overlap the ends of the strap by 1/4", wrong sides facing up. Stitch down the center of these lapped edges.
Now, the trickiest part. You need to fold the straps over to completely enclose the seam allowances. Stitch down the folds to hide the raw edges. (see illustration for how to fold)
Now my top fits perfectly. I'm very much in love with it!
PS - This seam finish works best with woven fabrics as knit tends to stretch out of shape during stitching the folds over the seam allowances. I used this stitch to shorten straps at the shoulder seam because I didn't want any raw edges in a visible area. I also use this seam when making reversible garments.