Tools:
Drift to remove the old bits of pin
2lb hammer
Plumber's pliers, channel-lok or similar with long arms
First, make sure there are no remnants of the old roll pins in the holes, top and bottom.
Make sure you are in a place where the wind cannot blow the door, or tie it open. The hinges are unforgiving.
Gently tap the roll pin in from the top, until it is located partway through the hole.
I used a piece of wire threaded through the center of the roll pin, and down through the roller, out the bottom to locate it more easily.
Tap the roll pin through and locate it into the roller. Hammer it through until flush, taking care that it makes the lower hole square, too.
With the door open fully, hook the spring to the body point, then with the pliers, pull it up over the lever arm. (Makes it sound easy, but you need a VERY tight grip).