I learned to upholster when I took a sculpture class several years ago...that being said, it's really not that hard if you're reconstructing something...
Just make sure you have a sturdy-weight fabric, and use the old upholstery as a pattern.
I usually start by stretching the material across the longest part, putting in a few staples to tack it down. I work along the edges, leaving a bit of allowance at the corners (say...2 inches on either side of the corner) to make a nice smooth bend.
I do the corners last, of course.
For the slipcovers, we just laid the fabric overtop, and pinned it (right side out.) leaving a bit of seam allowance.Then we sewed the right side, inverted the corners and sewed them, and hemmed the edges.