Jack, trial and error, blood sweat and tears.
In the end the best way seems to be that you can angle the mirror housing and line up the first (bottom rear looking forwards) and maybe even second (bottom front ) clips by eye then do the last one (front top) after getting the first two on.
I do recommend tethers, they have saved me a number of times already, in fact i actually have two tethers, one for the whole mirror/indicator housing made of wire and a second just for the glass made of fishing line, both have shown they work. (I live up a steep bumpy hill with a road made of concrete poured by the neighbours ten years ago...)
Good luck, I made sailors blush with my language the first time I tried to get them on before I got the knack of it