How to make narrower a pair of pants?

It depends on the fabric - how bulky the seams are etc. You've not said what the fabric is.

(1) Turn the trousers inside out and lie flat.

(2) Decide how narrow and what shape you want your trousers to be. (Remember the actual shape / width will be measured from the outer seam stitching to the new position on the inner part of the fabric).

(3) Use some tailors chalk to chalk the new position for the seam onto the fabric. The chalk line needs to start from the original crutch seam position and then gradually shaped down the fabric to the new position on the hem. Do this for both legs.

(4) Tack (= sew using double thread and big long stitches to hold the fabric in place) along the chalk lines right the way through all the fabric (front and back). Measure 1cm from the chalk line to the edge of the fabric, and chalk a second line from the hem up to the crutch. Using fabric scissors, cut off the excess fabric along the second chalk line (there'll be nothing to cut off at the crutch). Do this for both legs.

(5) Use a sewing machine, sew along the chalk lines from the hem to the crutch. Overlock the edges together on an overlocker. Alternatively, use a zigzag stitch with a very narrow width near the edge of the fabric.

Rosie
October 2013