Thanks to dieseldave for this.
Pease bear in mind that a curtain needs to be tensioned BOTH horizontally AND vertically…
Below is my attempt to give you the basics…
To open a curtain, first you need to get rid of the vertical tension.
To do this, you open the “buckles” by pulling downwards on the strap as in the pic…
Then, lift up the buckle and unhook the strap from underneath the deck…
(Have a look underneath to see how to put it back later 
)
Do this, from the front, as many times as you need to gain the opening that you need…
Then de-tension the curtain horizontally by pulling the (smaller) lever at the bottom of the tensioner towards you…
Then unhook the front pole by lifting it upwards…
To close a curtain, pull the curtain pole back to the the tensioner and engage the pole in the slot…
You might need to turn the tensioner to engage the pole in the slot…
Then push the small lever all the way away from you…
Then use the larger of the two levers repetitively to tension the curtain horizontally… it’s a ratchet
Then re-engage the vertical tensioners and snap the buckles shut…
Some other tips:
Depending on the maunfacturer of the curtain system, there might be a horizontal tensioner at either OR both ends of the curtain.
You don’t always need to open the whole of the curtain, but that depends on how much you’re going to load/unload and the type of curtain tensioners.
If you mention that you’re new to curtainsiders, I don’t think most drivers would mind helping if you ask them to show you.
As ever, I only try to give new guys a head start, without too much technical stuff. 