Ross Kirkland
It can be tricky, but if the door is a light weight moulded door, then should be fine. Calculate the wall opening width by adding the width of the door + 6mm hinge clearance + 2 x door liner width + 10mm liner clearance, calculate the height from finished floor/threshold by adding 15mm + door height + 3mm hinge gap + 1 x door liner width + 10mm liner clearance. Mark opening on wall and take care cutting out plasterboard with a handsaw (check for cables and pipes first). Push back the cardboard infill to allow pieces of glued timber the same dimension as the wall void to be pushed in to the wall flush with the opening, then gently screw through the plasterboard into these timbers to fix in place, complete this round all 3 sides. Then you can fit your door liner as normal and secure by screw (not nail) the liner into the opening; once packed plumb and level, sometimes a little low expand foam into the gap between liner and wall opening will help to firm up the frame but don't use regular foam or go crazy with it as it can and will move or warp the liner.
