....mark this position on the central pulley and engine block
...now release the tentioner bolts (*)
and remove the belt(*)
..when you do that(*)
the right pulley (photo-left)
will 'wander/turn' out of position
when you remove the belt,
as it is 'riding' on cam 'slope'
(witch is under the influence of the valve springs).
one thing you can do to calm your nerves
and keep it in position:
put the new belt loosely over the pulley and
...holding the pulley in the CORRECT POSITION (marks aligned)
'strip' a spanner to the frame,
...do not use the spanner to turn the pulley. Use your hands
...then 'lock' it with the spanner.
(make absolutely sure
you did not turn (release) the bolt)