If you're familiar with sewing and depending on how much needs to be altered you may want to takle this. Taking in a dress at the waist or hemming up something is relatively easy. If you're talking about altering anything more than one size, you're basically going to need to take the garment apart and 'custom' alter it to the person--this is not an easy task and will take some skill and time. If this is the case, I recommend purchasing or making a new item that is the right size.
When taking apart a ready-to-wear garment, note width and finish on the existing seams and hems to know what to do as you put it back together