Here is a method of mainspring installation that might be novel. With the usual methods, the hardest part is the spring compression, and maybe not getting hurt.
This method is SAFE, easy, and takes no special tools.
Before installing the spring or guide in the frame, I assembled them to a compressed state by this method.
I placed the guide inside the mainspring, and by hand, hooked the end over the nearest coil inside the spring. Then using a jeweler's file handle, screwdriver shaft, or similar thin rod to turn or hold the guide rod's plug end, and gripping the spring with my hand, (for tender hands, you could use gloves or a piece of leather,) I worked the guide hook up the spring coils, rotating the spring and working the hook up the coils as it were threading a screw. It became difficult near the top, but by making sure the spring and guide were rotating at the bottom where the plug is, and alternating with moving the hook slowly at the top until it had come to the top of the spring and the entire spring was compressed under the guide rod's hook. The hook keeps the spring comressed by degrees this way, and avoids the potential damage to skin or eyes of a sudden release of compressed tension from an untrapped spring.
> > > > > > turn the screwdriver / guide rod, while holding the spring
> > > > > > a few more turns....
Then it was easy to place the spring and guide assembly inside the frame without fighting the spring, positioning the hook to nearly contact its home on the lever. Turn the gun upside down on its ears, on a smooth hard surface that will not mar the frame finish. Now with a stiff metal rod, ( that jewelers screwdriver shaft works well ) inserted into the guide rod's plug end hole, push the guide rod a bit more comressed, and manouver the hook onto the lever. Making sure the hook stays hooked on the lever slowly release pressure on the guide rod until everything settles into place correctly.