If your waistband is in one piece then you have the chose of either removing the waistband, and then shortening it or cutting the waistband in half at the back of the trousers If you do this then ensure you ware taking the trousers in enough to give you a seam allowance on the two halves of the waistband.
You will need
Unpick the waistband around the back seam of your trousers and unpick the seam at the centre of the back of your trousers from about half way down from the wast band to the centre of the crotch seam.
Place the waistband on top of the trousers and mark where the back seam meats the waistband. Add a seam allowance and trim the excess waistband.With right sides together sew the waistband back together.
With right sides together sew the outside of the waistband to the back of the trousers. Press and then stitch carefully on the outside of the trousers just under where the waistband meats the trousers. This stitches the inside of the waistband to the inside of the trousers.
Things to think about.
This is a very quick way to take in your trousers. However there are some things to be careful of.
Ensure the back pockets are not sewn over.
Within the waistband of the trousers you will find either a cotton tape or petersham waistband. This is what stops the waistband stretching out. Ensure this is stitched together when you sew the two halves of the waistband together. Also if this is lose in the waistband make sure it does not twist. Otherwise your trousers an become uncomfortable.