I buy skirts and dresses from vintage/thrift stores and alter them all the time! It can be really simple.
If you find a skirt with a simple pattern (not pleated or anything like that) it is usually just a matter of folding part of it over on the inside and sewing it up. I've even done this by hand on a couple of t-shirts.
Once I found a wrap type skirt that buttoned up the front that was a little too big on me, so I just took off the buttons and sewed them back on an inch to the side.