"To miter the corners" means to cut or shape the corners at a precise angle (usually 45 degrees) in order to join two pieces together neatly and seamlessly. Full definition
It features mitered corners, hidden welds, quality mesh and hand - scrolled legs. (potterybarn.com)
Include some additional working length for mitering the corners around openings, if neccessary. (interiordecorating.com)
We did mitered corners and glue and nailed each piece to the door and made measurements as we went so that the fit for each piece was accurate. (coloursaturatedlife.com)