King
Make sure the cill is spot on level wise, use a decent size level not a Mickey Mouse diddy one. Once level, run some silicone and/or some foam along the brick/blockwork, some prefer to screw the cill down if you do don't over tighten the screws this will make the cill dip, check the cill with the level again if you have fixed it down.
Run a 'U shape' (viewed from in) bead of silicone along the top of the cill, place frame in and wedge in place or get someone to hold. Frame needs to upright and plumb each way, watch when you open the doors to drill not to get any dust into the running track, make sure the screws are counter sunk in to avoid effecting the doors from running smooth. Once you've fixed sides and head, foam the gap from the outside and have a cup of tea!!
Next bit is the glazing, start the opposite end to where your door is, each pane needs to be packed (toe and heeled) to suit the sash, make your way along until u reach the door itself, again toe and heel, adjust the keeps to get it working nicely, there should also be an adjustment on each of the hinges to close up the gaps if any or to move the doors over if the main door is catching on the frame..... Vacuum out the track, clean, wd40 the track, Roberts your mothers brother (hopefully)
Good luck