The waist band of jeans are tough to alter easily. The quickest method is to do what I call “he’s going to grow into them some day”. This happened when my son was about eight. His grandma had bought him a great pair of black jeans. However, they were regular and he was a super slim. What I did is make a floating interior waistband of elastic. I took inch wide band of elastic and cut it to his waist size less a about an inch. Then, I tacked the ends to the waistband inside the jeans to left and the right of the button area. Then I tacked them equal distantly around the inside in about four spots.
You can see that the elastic lays inside the waist band and is tacked at each end by the button and buttonhole and the interior tacks.
The down side to this type of mend is it can create floating clown pants. They will stay up but there is a lot of gaping. If you wear your shirts long, it works fine. It works best, as I said, for people who will soon be growing into their clothes like kids or pregnant ladies.
The second way to bring in a waistband is to make darts. This works best if your pants are just a little big and the waistband is rather wide.
Here I have taken the waist band and sewn a dart. A dart starts wide and ends in a point. At about.com I found this good visual of how to sew a dart. Basically, you pin two layers of fabric together in a line that tapers to a point and sew the two layers together. The downside to this mend is the darts can rub against your body and that can sometimes be uncomfortable.
I think the best way to alter the waistband would be to remove completely either by picking it apart or by cutting it off, and creating a new waist band out of other fabric. I have never done this, but I think it could look really cool, especially with contrasting fabric.
She also asks about the inseam. The inseam can be brought in very close to the zipper, but honestly, only that far. Shortening the zipper is a big pain and I wouldn’t recommend it. But if there is enough fabric, it can be sewn in the same way as the outside of the legs.