Summarize:

Turn your jeans inside out and put them on. Pinch the waistband on each side until you get the right fit in the waist. Try to pinch an equal amount on both sides so that your jeans will sit evenly after the alteration. You can secure the pinched fabric with a large safety pin to help you as you proceed to the next step. Carefully put the pins in the waistband on each side where you have pinched the fabric, as close to your waist as possible to keep the jeans snug. Be careful not to pin your finger. Keeping pinning down the sides of the jeans where you can pinch out loose fabric. Pin as far down as you would like, depending on how you would like the jeans to fit. You can pinch and pin just along the waist, down to the mid-thigh, or even all the way down to your knee if you want a extra skinny fit. Carefully take your jeans off. Sew each side of the jeans along the pinned line. Use a sturdy denim needle, a longer stitch length than normal, and higher tension. Go over the stitches again with a backstitch (reversing back over your stitches) at the beginning and end to secure the stitching in place. Try a stitch length of 2 and a thread tension of 4 to start. If that doesn’t work, you can easily take out the stitching with a seam ripper and try again with different settings. Don’t be afraid to experiment until you are happy with how your seam looks. Try your jeans on again and check the fit. You can always take your stitching out and try again if there is something off. If you are happy with the fit, but feel like excess fabric inside the jeans is too bulky, you can cut it out. Leave about a 1⁄4 inch (0.64 cm) border outside the stitching to prevent the fabric from unravelling. Otherwise, you can leave the fabric in. You can also fold the excess fabric to one side and sew the end down so it lies flat inside when you wear them.
Put your jeans on inside out and pinch the waist at the sides until it fits. Secure the excess fabric on both sides with straight pins. Sew next to your pins with a single straight stitch. Turn your jeans right side out and try them on.