Not sure what the official answer is, but currently training roses up a pergola, and looking good so far…
I selected 2 stems from the rose and wound them in opposite directions around the pillar. This must be at an angle of 45 degrees or less if you want flowers on the pillars and not just on top. This has taken a couple of years so far for us - I think this year we will start to cover the top of the pergola.
Prune out all other strong long stems - just keep the 2 long ones.
Also, plants (mostly) grow upwards. So I temporarily encourage the stems to grow upwards, and tie onto the pergola so it’s going up. When I have enough length (1-2m?) I untie, wrap it around the pergola, and then re-tie and repeat. It won’t grow very well at 45 degrees, but will happily upwards.
patience seems to be the key - think years not months!
The risk as I understand it is no flowers on the pillars, just on top of the pergola where you can’t see them. My pillars are covered in flowers at the moment, and seems to be worth the wait!
but I’m no expert - so good luck!,