Altering an existing pair of trousers to give them a flare is a great way to personalise your style. In only a short amount of time, you can change a pair of trousers that no longer suits your style into a totally different look. With only a sewing machine and some extra fabric, you can easy transform old trousers into new ones. Try using contrasting fabric to stand out, or similar fabric if you would prefer the flares to blend in.
Lay your trousers on a table and cut the outside seam on either leg up to the knee. Use scissors to cut the stitched seam, taking care to only cut the thread and not the fabric.
Get a piece of fabric you would like to sew in to your trousers, and cut a long triangle out of it, with a base of about 3 inches and a point that extends up from the cuff to the knee. Changing the width of the base of the triangle will change the amount of flare your trousers have. Experiment with smaller or larger triangles before sewing to ensure you get the amount of flare you want.
Turn the trousers inside-out and pin the fabric to the opened seam of your trousers.
Sew along the edge of the triangle and seam, connecting the triangle to the trousers. Do the same with the other side of the triangle and the other seam. Repeat this step for the other leg as well. Hem the bottom edge of the triangle if you want a clean bottom edge.
Try to select fabric for your triangle that is the same weight as the trousers you are adding it to, to have them flow nicely.