Turn crankshaft until 12 degree before top dead centre mark which is about 31 mm before 1-2 mark on flywheel.
Peal back the locking tab and undo the bolts holding the pulley to the camshaft, mark the hole that the pin dowel is in (as a confidence check).
I use a digital calipers as shown in the tappet ball socket and measure to the carburettor holding stud.
Turn camshaft until 13 thou reads on the caliper. You will need to return the follower back onto the shaft by pushing with finger! The purists may say that it should be less than 13 thou because of the fulcrum action of the rocker which is why you see the guage reading 11thou. The acceleration of the cam is large at this point so will only make a degree difference if you are a few thou out. Earlier opening may result in popping back through carb; later opening loss of power due to insufficient charge.
I then mark the position of the cam as shown. The mark lines up with the split in the crankcases. May be worth doing 3 times to check position by revolving crank through 720 degrees each time.
Then position pulley onto camshaft so that bolt holes align and fix dowel into hole that best lines up with that in camshaft.
This can be done with timing chain attached. The marks on the pulleys should line up (usually two pop marks near the teeth).