Hi there, the most important thing is to make sure that the base cill is set absolutely level, firstly using a spirit level and if required some packing material (plastic purpose built packers in various thickness's are available or even use slithers of wood to pack up and set the cill level. one you have the cill level. I would suggest lifting the cill off without disturbing the packers, and then putting a mortar bed between the packers to make sure that the cill is supported evenly. Dont forget to make sure that if you have had to lift the cill to level it that you keep the sufficient height to fit the door into the aperture. You can then offer the framework in and then fix to the side walls. again make sure the each side is level (sideways and up the front), and if needed put packing pieces behind the frame where the screws are going nto the wall to make sure that the screws dont pull the framework (a) out of level) and (b) put a bend in the framework.
If you do this the doors should just drop into the frame and work like a dream. Good Luck!!
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