After 40 odd years trying this still presents me with a challenge.
It is probably the worst job to do on a preunit twin.
I dont think there is a foolproof way and I disgarded my pushrod comb years ago as it was quite useless.
The first issue is the limited space between the top frame tube and head can make it difficult to engage the four holding down studs into the holes in the head. This is worst with the alloy head 650s with long studs. The top inlet inspection cover stud was originally made with machined flats to ease its removal to increase the space.
Having overcome that obstacle I favour the elastic band method to hold the rockers in position. I put a bit of grease on the lower end of pushrods to hold them in the cam follower.
With an alloy head I cover the holes shown in the photo with a bit of tape, just incase the nut is dropped whilst fitting it, it can be a bit of a fiddle getting it out.
Fitting the sleeve nuts on the alloy head can also be a bit of a fiddle, getting the threads on the nut to catch on the stud.
Having positioned the gasket and applied your chosen sealant, lower the box onto head and guide the pushrods into the rocker arms using your fingers. Sometimes the pushrods will be caught on the front edge of the inlet joint. Having fitted the pushrods, probably after several attempts, and avoiding force insert the four long bolts and gently tighten a bit and add the other bolt and the four nuts. You may need to rotate the engine to allow easy tightening.
Having finally tightened all the fasteners you can set the valve clearances.
If using thick modern gaskets, which might come in blue or orange or yellow materials, expect them to settle after a little use needing the fixings being retightened and clearances to be reset.
Keeping the oil in can also be a challenge but that is another tale.