Don't you hate it when you find a dress that you think you are about to fall in love with, but then find realize it's not your size or it fits weird? I am so thankful that knowing just a little bit of sewing can go so far and how it can put potentially missed opportunities straight into my closet. I fell in love with a pretty peachy pink lace dress at H&M, but it was very unflattering on me. It was already an XS, but I was looking a little frumpy. The material felt great and the quality was good, so I took a chance. And, I'm glad I did! That worked out so well that I bought another oversized and unflattering dress at Walmart to take in.
As you can see here, I took in more around the waist. Once the new seams are sewn, try it on. If it fits, then trim off the excess fabric and press the seams.
If you don't have a dress already in your closet to use as a guideline...
Then, WEAR your dress inside side out, pinch in the excess fabric with your fingers, and mark your potential side seams.