By now we're all familiar with the use of the STC3064 cam kit and cutting off the extraneous bit of metal and we know these work well, BUT...
Getting the spring onto the plastic cam AND into it's correct locations can be a long process and hard on the fingertips and patience.
Here's the (well, an) answer- cable ties.
First the easy one. Use the first cable tie, with key in lock, to hold lock tab out of the way, like this:
Now for the spring. The problem is holding it under tension and connecting all the components at the same time, traditionally requiring 3 hands. The solution is to pre-tension the spring and use a cable tie to hold it under tension.
To do this (and I apologise that I don't have better photos, but again that would require 3 hands) put the spring onto a handy sized bar, screwdriver, whatever and prepare a small cable tie in a loop. put one end of loop under small arm of spring and press spring/ bar/ cable tie combo on table top.
Bring long arm of spring over into its tensioned position and tighten cable tie to hold that tension. Note you don't need as much tension as pictured.
Spring is now compressed and held compressed by cable tie.
Remove from bar and assemble handle/ cam components. They just drop into place easily without battling the spring.
Gently move handle to ensure everything assembled correctly, insert new roll pin, cut cable ties, spring releases.
Take other cable tie off lock arm
Pics of in progress solution: