Hold the try square by the stock with the blade pointing forward and press it firmly against the item you wish to check so that the blade rests on the top edge.
Check to see if any light is visible between the stock, the blade and the item. If there is, the item is not square.
Lay the blade of the try square flat against the item you wish to cut. Push the stock up to the edge and hold it firmly to ensure you mark a true right angle.
Use a pencil or knife to mark a line on the item and follow this while cutting.