There is quite a knack to it
I fit the top first and then I have a ling flat blade screwdriver with about a 5mm steel part
I fit the stand and hang the spring on the top mount
It "Seems" to be easiest to do this at about halfway between up and down (Less spring tension)
I insert the screwdriver through the loop, then fit the flat blade under the springs nub
And I push the spring down the screwdriver shank as far as I can and keep tension on and start to push down and extend the spring
AS the spring extends keep control of it as best you can! As it passes level it starts to slide down towards the nub go easy on the downwards force now
Keep control of the spring and don't let it slide under it's own volition or it will slide down bang off the nub and either fly off or become jammed!
You have two important things to monitor while you are doing this !!
1) Keep that screwdriver tip under that nub!!
2) Keep the Spring under control, Don't let it slide of its own accord
So if you have the spring down by the nub and the screwdriver tip still under it? With a bit of luck You can now see how to manoeuvre the spring causing the spring to follow your movement and go around the nub as you slip the screwdriver out
IT may be best to try this first with a weaker spring or couple of smaller ones linked or even your own one packed with washers to reduce spring tension
The method does work and its quite favoured in the Scania workshop where I was, for fitting Brake shoe springs and you get quite proficient at either fitting the spring or going back to the store and asking for another one as the one you had launched into a low orbit