Despite whichever ways the H&S bods want it doing AFAIC it’s clip first and last as it’s too easy to forget it, then follow the rest of BLACK. I never secure the load or close the curtains at the same time I’m picking up the trailer. I treat them as two separate tasks to avoid mistakes trying to multitask, as every woman knows that us blokes can’t multitask 
It’s wise to stop, get out and have a quick walk around the trailer before you pull under it, as it enables you to check that the parking brake is on, spot obvious defects and faulty tyres before you pick it up, and therefore saves you the anguish of early morning grumbles and under the breath curses of having to drop it back down when you discover it is fubar