Recently, I have had the very double-edged sword predicament of having lost weight through exercise and nutrition (good!) to the point where all my clothes don't fit (bad!). I was previously an RTW size 6-8, but now I'm finding a lot of 4s too big, especially pants. I would like to salvage as much of my large (2 closets + dressers) wardrobe as possible and have found the cost of outsourcing the alterations a bit prohibitive for the sheer amount of items which need fixing. Plus, I can sew, right?, so I would feel a little guilty about doing that.
My Mom taught me when I was a teenager and I've been sewing off and on over the years. I'm mostly strictly a pattern sewer, but I have done a little tailoring on some of my projects. Skill-wise, I would classify myself as a "determined beginner/intermediate". The most complex alterations I have made so far, though, are only shortening hems. When faced with changing sleeve lengths or taking in waistbands, I quiver a bit. Especially with lined items. *shudder*
I've found tons of information on tailoring, but not much on changing a finished garment. Here is the *single* book I have found so far and even it looks a little out-of-date and iffy: