The new frame will not require extending into the brickwork, so fill those gaps with a cement mortar mix. Under the bottom (the sill) there should be evidence of a damp proof course. Check the condition of the damp proof course and replace it if there are any signs of damage. If you cannot see a damp proof course below the sill, fit one before replacing the frame.
Once the old frame is completely removed and the gaps have been filled with a cement mortar mix, check the surround for old nails or damaged render and repair before proceeding.
Fitting the new frame
Use waterproof wood adhesive to assemble the new frame. Use on all joints and screw them together. Insert the frame into the opening using wedges to fit under the sill to push it up against the lintel. The sill must be perfectly square and a spirit level should be used to check the sill is completely horizontal before fixing. Next use the spirit level to check the angle of the jambs, which should be vertically upright. Use wedges to push them into position if necessary. Keep checking with the spirit level and adjust the wedges if necessary. The final check of a perfectly square fit is to measure the two diagonal distances (top corner to opposite sill corner). If they are not exactly the same, you must adjust the wedges and continue checking the angles before proceeding further.
Once you are satisfied that the frame is perfectly square, the frame can be secured to the brickwork. Check the positions of hinges and locks before drilling holes for fixing the frame to the brickwork. Drill holes through the frame and brickwork. Avoid these sites when fixing the frame, but for extra security, drill a hole just above and below where the position of the intended lock. Countersink the frame holes and tap in fixers. When fixing the screws, check with the spirit level that the frame does not become distorted due to driving the screws home too tightly.
If the door is in an exposed position and is not protected by a porch, it must be fitted with a weather bar. This will prevent rain running down the door and underneath. The weather bar is a narrow plastic or metal strip set into the sill.
Finishing the new frame
Hang the door and make any necessary adjustments to the door or possibly minor adjustments to the frame. Cover all screw heads with matching wood filler. Use the cement mortar mix to fill the spaces under the sill. Use an exterior sealant to completely seal the jambs (sides) and the top.
