Good evening Terry,
I always fit the boiler lagging sheet temporally and carefully set out and lightly scribe the line of the hand rail also marking off the positions of the stanchions, take off the lagging sheet, drill through and it's a simple job to fit the stanchions nutting them on the back, but firstly checking that the hand rail material will pass easily through the stanchions. All that's left is to fit the lagging sheet for final.
Regards,
Bob