If you try to put the spring back in by holding the plunger and pushing the spring into the shell, it will indeed be an adventure. To make it easy-peasy, try putting the spring in this way:
Assuming you're right-handed, hold the shell in your left hand and lightly pinch the spring between your thumb and index finger. Then grab the spring with your right hand about an inch above the shell. Push the spring in with your right hand while releasing it with your left. Then pinch again with your left and repeat until the spring is completely in the shell except for the plunger. Finally, grab the plunger with your right hand and push it into the shell.
Just reverse "right hand" and "left hand" if you're left handed. Takes about 20 seconds to install the spring.