I depends on where they are too big?
If the waist is the only problem, start on the back waist band and make to darts (evenly spaced each side of the center back)and run them down a inch or two to take out the slack. When you sew the darts start at the top of the waist band and sew down to the point of the dart. You then press the dart (with an Iron) to the center of waist band. Or you can cut the dart ( if you cut then if you gain your weight back you can not let it out).
Are you using a sewing machine? make sure you change your needle for jean material or it may break