Oh, it can be a bugger!
But once you get the hang of it, it's not so bad.
First, insert the top of the spring under the roller of the hammer. Then, apply pressure to the base of the spring to slip it into the cut in the grip frame.
If you can't apply enough pressure with fingers alone, use wide-nose pliers to GENTLY compress the spring a bit until it lines up with the cut in the frame. Then slide it sideways into the cut.
I have an Uberti copy of the 1858 Remington. Sometimes I get it in by hand, other days it requires gentle comopression with the pliers. :roll: