Let's see if I can describe my method.
Hold the roller arm where it will be when properly assembled. (No spring or pin at this time).
Swing the arm back towards the center, so it is upside down.
Put the spring between the roller arm and the base bracket.
Slide a thin round shaft through the holes in the arm, base bracket and spring.
Flip the arm over (the smaller shaft gives you room to wiggle things around to get past the stop that is built into the side of the roller arm.
Slide the retaining pin in as you remove the smaller shaft.
I use a #1 Phillips head screwdriver for the small shaft.
You may need to finesse the position of the small shaft and spring to keep everything together as you flip the arm over.