Remove the window panes from the metal plates that hold them in position by flattening one side of the plate with your hand. Put them out of reach of children. Remove the screws holding the metal plates and put them in a safe place; you will need to reuse them.
Measure the old window frame. Put the crowbar between the window frame and the wall and remove the whole frame even if only one side is ruined. Saw any stubborn nails if the frame will not come off.
Measure the length and width of the window. Transfer these measurements onto the plywood you'll be using to make your window frames.
Saw the plywood into four strips the same size as the existing window frame. Measure 3 inches from each end of each strip and mark with a pencil. Cut the strips from the end to the marked points making a 45-degree angle at each end. Join the strips together at the 45-degrees with wood glue.
Set this window frame in the position you had removed the previous one. Fix it firmly on the wall with at least three evenly spaced nails on each edge.
Screw the metal plates on the newly fixed window frame. Place the window panes you had previously removed in the metal plates. Twist the flattened ends of the metal plates so the window panes are held firmly in position.
Paint the new window frame.