It's hard to know since I don't know what kind of waistband it has. Sometimes, just bringing in each side a little (even amounts) and stitching them would suffice. You can do this with a hand needle if you can't sew...just sew the waist in a bit on each side. Use pins to mark how much to take it in. When it fits, make the permanent stiches. If it's an elastic waist, measure a piece of elastic just a tiny bit bigger than your waist is now. Take your skirt and make a tiny slit at the waist inside, where it won't be seen. Find the old elastic. Snip it and pull it out through the slit. Take a safety pin and hook it to the end of the new piece of elastic (and close it up). Feed it through the slit, bunching it all the way around. When both ends of the elastic meet up, stitch them together securely. Then, stich up the little slit, and you'll be set.