Remove the hinges from the door by unscrewing the screws that hold each hinge to the door. Most doors feature at least two hinges: one at the bottom and a second at the top. Carefully pull the door away from the frame and set it aside.
Measure the width and height of the door. Purchase four wood boards -- two the same width and height as the sides of the doorway and two the same width and height as the top and bottom of the doorway.
Set one of the longest boards against the side of the door. Place a drywall nail against the bottom of the board and hammer it into the door frame. Hammer the nails every four inches along the side. Repeat with the other three sides of the frame.
Pour the mortar into a plastic bucket and add enough water that the mortar has a thick consistency. When you add the right amount of water, the mortar should have a texture similar to that of peanut butter. Follow the manufacturer’s guidelines to achieve the correct ratio.
Use the trowel to cover the bottom of the door frame with a ½-inch layer of mortar. Apply ½ inch of mortar to the bottom side of the door frame. Use the same amount of mortar on the side of one brick and set it into the mortar, making sure it sits flush against the frame.
Arrange a layer of bricks across the bottom of the door frame, using mortar on one side of each brick and placing a ½-inch layer on the top of each one. Keep stacking the bricks until you fill in the door frame. Some bricks might need to be cut to fit.
Go over the bricks with mortar, filling any gaps or problem spots. Apply the mortar to the empty spots with a trowel and smooth it down. Let the mortar dry overnight and check for any contracting. Fill the gaps a second time and let it dry.