Refashioning has been a tar pit for me. I used to spend all kinds of time thrashing around until my dream of inexpensive cutely fitted items sunk out of sight.
But my slipcover project must have burned some new synapse in my brain. Pin seam after pin seam has taught me that the human body is nothing more than an oddly shaped cushion. Thus, above you see my first successful alteration -a resized thrift store cardigan.
The secret - unleashing the power of the straight pin.
We start simple here with a top that already fits in the shoulders. (Unfortunately I don't have a "before" photo to indicate its former voluminous size elsewhere.) We will tackle the scary world of shoulders another time.
Button the sweater up, turn it inside out and pin the sides to a size that fits you. To save some pinning time, I sometimes start by laying a top that already fits over the larger top. I use the smaller top as a pattern and pin my sides accordingly.
My pin line is indicated in pink. Note that you can't just stop at the sides. Continue into the sleeve, then gradually angle back into your original seam.
Try the top on inside out and adjust your pins if needed. You don't want your buttons straining (too tight) or your sides droopy (too loose). Use a measuring tape to check that you are taking it in the same amount on both sides. Take your time with the pinning. Get it right before you sew.
Here you see my seam. I used a long straight stitch. There are special stitches for stretchy fabric, but I have not gone there yet. My straight stitch worked fine.
Notice that the new seam goes basically parallel to the old seam. Don't get too curvy. Your top will stretch around you anyway.
Here you see the seam gradually joining the original seam in the arm. Sew your new seam all the way to the end of the sleeve, right over top the original seam.
You want the edges of your fabric to match up. Here I'm a little off.
Then, if you're not lame, you trim your seams and finish the edges with an overlock or zigzag stitch.
Or you leave them as is because they look and feel OK and nobody would know if you hadn't posted an inside out photo of your cardigan on your blog.