Teen

You can accomplish this by lowering your feed dogs. Feed dogs are those teeth under your presser foot that feed "guide" fabric under the needle automatically. When you lower the feed dogs, than you have to guide the fabric manually in the direction you want to go. This is called free-motion. Most machines have the ability to lower the feed dogs either by a switch or button. I gotta warn you, though...this takes practice because you now have to move fabric at the correct speed to ensure your stitch length stays uniformed. So, practice on scraps first... If there is another way to do "stitch sideways" I am not familiar with it.....
And, since it's a pant leg, this may prove difficult without opening a seam.
Welcome to the board...