Timber Plugs
Timber plugs will now need to be inserted into both sides of the brickwork, so that when the window unit is fitted between then, it's a simple  matter of fixing the frame securely in position by nailing through the side linings into the plugs.
With the frame removed, three brickwork mortar joints will need to raked out, top, middle and bottom on both sides. You may find this easier to do by using a hammer action electric drill with a masonry bit fitted to drill out the hard mortar. The plugs need to be positioned about 75mm from the top (of the frame), and the same from the bottom, with one in the centre. Obviously, the exact position is dictated by the mortar joints.
The timber plugs will need to be driven into each of these joints using 15mm thick timbers about 75mm wide and left protruding about 80mm each.
With the installation of the plugs complete, they will need to be cut to the correct length so that the horizontal space between them is equal to the width of the new frame (and the thickness of the two side linings). This is achieved by accurately transferring a mark from  the brickwork that was previously placed there, onto the top two plugs, and because linings are to be added onto each side of the frame, the thickness of these linings will also need to be added onto both plugs, effectively increasing the distance between them by 18mm on each side.
The top two plugs can now be cut to length, ensuring that the cuts are parallel to the brickwork. Next, place a spirit level vertically onto one of the cut top plugs, so that it also covers the two plugs below, adjust the level so that it is perfectly vertical (plumb), and mark the two plugs below. Do likewise on the opposite side, but before cutting, double check the measurements between each plug to ensure that the width of the gaps between them are all the same, and equal to the width of the frame, plus the thickness of the two lining pieces.
As you may appreciate, these measurements and accuracy of the cuts are essential if the window frame is to fit correctly in it's predetermined position.
With the plugs cut, fit the window back in position onto the plywood packing's up to the frame width marks on the brickwork, and central to the second mark on the sill, and place a spirit level onto the internal face of the frame to ensure that it is vertical, and prop or wedge it in this position so that your hands are now free to measure the window linings.
Thought the frame is positioned correctly, (level and plumb), it's very likely that the walls are not, due to years of settlement and movements of the brickwork. This difference between the new frame and brickwork could lead to timely adjustments having to be made later by adjusting and scribing the linings to match the internal plaster-line. To overcome this problem, the linings can be adjusted to meet the plaster-line first, and because the lining is designed to travel passed the side of the window frame, and be fixed onto it, it's an easy process at this stage to set the lining position independent of the frame.
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