This is where Rodico is your friend. Put a small piece just behind where you want to drop the spring in. You can then stick the spring to the Rodico, and manoeuvre it straight, and get the right angle, before sliding it down in to place. I find sharpened pegwood sticks help.
Some will tell you to put it in with tweezers - but it's not easy. Replacing these springs is one of the things I struggled most with (apart from wrestling hairsprings of course !)
Also when opening the springs, If it looks a bit loose, I will use a bit of Rodico to to make sure it's not going to pop out of it's slot.