Measure the height you need and the opening left to right.If using 2 doors divide this amount by 2 less 1/4 in [1/8 per side ] for the hindges.Use 3/4 in Birch plywood,available at Home Depot or Lowes. Let them cut it if you don't have a table saw.You can also add moulding around the frame of the door. You will need a miter saw for your 45 degree cuts.Not to difficult. Oh,don't forget door knobs!