This guide to installing your Yamaha QT50 kick start return spring when rebuilding your engine was originally posted in this forum thread. The question comes up periodically about how to do this and it is really hard to explain it without pictures so here they are.
The first pic shows the short bend of the spring in the proper hole in the kick shaft quadrant. The hooked end will need to be rotated clockwise 180 degrees and hooked over the post in the lower corner AFTER the plastic guide is installed. The plastic guide has half of it recessed to properly seat against the edge of the kick shaft quadrant, as it rotates with the shaft when you step down on the pedal.
The last picture shows it all properly installed, ready to go. There's a lot of tension on this so don't poke yourself in the eye while attempting to load the spring.