Firstly when removing the old arms, mark the adjustable bolt on both sides with a black or preferably white paint marker. Run a straight line from the washer either side to the body work. This way when you put it back together you can bolt this back up in the same position.
Now remove the old arms and put them up on the bench with the new arms.
One at a time, remove both ends of the adjustable arms, then screw each end back on the same number of turns. This ensures both ends are installed with the same thread engagement. Now lay it flat on a bench and run a long bolt or rod through the bush end of the new adjustable arms and the corresponding old factory arm, so they are locked side by side. Then holding the other end of the adjustable arm from turning, turn the centre adjuster until both arms are exactly the same length, then nip up the lock nuts. Don't do them tight yet as you may need to adjust them slightly to get them installed.
When installing get both arms with all bolts engaged before tightening. If needed loosen off the adjuster on one of the arms to get them started then once torqued up reverse this adjustment to return to the length you started. When adjusting count in increments of 1/6th of a turn with the flat face down for each increment.
Be sure to torque the bolts to the correct torque setting (check the manual lol). If you do the smaller bolts too tight you'll risk snapping the heads off. If the big adjuster bolts are not tight enough they'll turn. Finally tighten up the lock nuts on the adjustable arms.
If you get the new arms the exact same length as the old arms and install them with the adjuster bolts correctly lined up to the marks you made earlier, your rear wheel alignment should be almost exactly as it was when you started. Now it's off to your local 'competent' wheel alignment specialist for a four corner wheel alignment. Remember to take your 2x 26mm spanners just in case they don't have them lol!
that'll be the Toblerone guide right 
