I think spring goes below the brass thumb turn pin to lock it in open or closed position , it only works one way round so you may have to turn it over .
Yes I know where it goes , but I’m don’t have one to show you and I doubt you would understand directions, hence the video search which produces exactly what you need .Did it not occur to you to mention the brand? As they all vary in design .
After you have assessed the spacing and number of spindles you can fix hand rail , offer in all the spindles then you can move them along one at a time and add your spacers ,