Just set the engine to the full advanced marks (about 35 deg BTDC) using the visual marks on the sprung Advance unit. Whack a couple of sparkplugs in the the 2 x HT caps nearest you #3 & #4 and make sure they're grounded on the head.
Now switch ignition on & twist the Trigger Sprung Advance mechanism by hand fully clockwise (push against the springs) and watch for the spark which should coincide with the unit reaching Max Advance

Simples
