Place the spring like this
Pretty much at 12 oclock in this pic, theres a small cutout. Align that with the spring, and twist 90 deg to normal.Try to keep the twisting centred, otherwise the spring will not fall into the ring of the name plate(so should look like below pic as well). After youve done all that, then you can put the screw on. Theres no need to partially insert the screw(prob makes it harder), and dont use dental floss as itll prob snap trying to overcome the spring tension.
This pic is for the RX100 shifter, but its the same sort. Now dont be confused why the spring is in the name palte and not on the lever as I directed above. Its just a visual aid.
I guess those who make STIs sound like they are fragile dont know what they are going on about. But its a good thing if you need a set of STIs for cheap!
Oh, I guess I should do a video on rebuilding these, as itll make it easier for everyone. Now, if I manage to get my hands on 6400 ones, then Ill do it.