Check the level across the threshold to determine if the hinge side of the opening is higher or lower than the lock side. If it is lower add packers on the hinge side to rest the door frame onto as you install it.
Mark the positions of the top and bottom hinges on the wall, hinge side only. Fix a plywood packer to the masonry at the top hinge position. It doesn't matter what you fix them in position with (panel pins, oval nails, screws, etc) so long as you fix them.
Pack out the bottom hinge position so that a 6ft level shows a true plumb when laid onto the two sets of packers.
Position and fix a third set of packers in the middle of the opening (half way between top and bottom hinge packers). This must be plumb to the others
Cut a rod which is a couple of millimetres longer than the door frame is wide. Using the rod position and fix three sets of packers in the masonry on the lock side opposite the hinge side packers.
Offer in the door frame and fix it to the masonry at the three packed points on the hinge side. The fixing must go through the packers and into the masonry, not into the mortar joints. The straightness of the jamb can be checked using the 6ft level and any packing necessary to remove barreling added as required. The casing must be plumb both on the inside of the jamb and when checked on the outside edge of the frame, regardless of whether or not the wall is plumb
Level the head of the door frame using a 2ft level. If needs be place shims or wedges beneath the bottom of the lock side jamb to deal with any difference in level of the floor.
Trim the rod you cut earlier to be the same length as the door opening (nominally 6mm bigger than the actual width of the door which allows a 3mm gap at the sides)
Fix the frame on the block side to the masonry through the top packers. It may need an additional couple of 1 or 2mm packers to make a tight fit. Check with the rod to ensure thst the opening is consistent
Plumb up the lock side jamb (level on the edge) and fix through the lower set of packers, leaving a bit of slack in the screw to take up. Check the distance across the opening using the rod and add extra packers as needed.
Repeat for the middle packer set.
You can sometimes check if your frame is cross legged by sighting one leg at an obtuse angle and moving your head towards the wall to see of the other leg is in alignment.
For door frames which are flush, or where this method of checking is impossible (e.g a door frame in a narrow corridor) there is an alternative method of checking for a cross legged frame:
1. Push a pin into both top corners and bottom corners of the door frame
2. Wind a thin cord or piece of string starting bottom left corner and run the string to top right, top left and bottom right in order. At each pin the string must be hard against the wood work. Check where the strings cross in the middle of the opening. At this point if the jambs are coplanar (in line) the strings should just kiss each other
3. Rethread the string starting bottom right corner and running to top tight, top left and then bottom left in that order. Again the strings should just kiss in the middle.
4. If the strings don't kiss and there is a gap then one of the jambs isn't plumb and will need to be checked for plumb and adjusted.
Notes
A "rod" in this context is simply a straight piece of 2 x 1 planed softwood, rip of plywood, etc which can be cut to length and used to check distance. Once made it is more accurate and repeatable than any measuring tape will ever be (as well as not being subject to human error)
Whilst a 6ft level is ideal, it is expensive. A substitute can be fashioned from a 3 or 4ft level taped onto a dead straight piece of 3 x 2 CLS, 6ft long
When screwing through the frame pre pilot and countersink the screw holes. If using 5mm screws to fix the frame in place, pre pilot holes at 5.5 or 6mm. This ensures that should you want to reverse the screw out of the hole several times whilst adjusting the frame it will come out without bringing a big chunk of timber with it