Right , safety first !! glasses and gloves!!
Get a quality glass cutter , using a straight edge and on the glass only side , carefully positioning the cutter exactly where you want the new edge, score it gently from top to bottom, making sure it is scored all the way , (you only get one chance at a single score so do it steadily!) then place the scored edge over some matchsticks (seesaw style) gently press and it will crack across the score line,
There are other ways /quicker ways but for a first time DIY that way is fool proof , try some practice ones , and the lines don't need to be straight they can be curved as well ... best of luck !