It's definitely a hassle that becomes less so the more you take the pistol apart.
My method:
1) Push down trigger bar and insert right side decocker
2) Position decocker spring in the post using my ceramic tweezers (no magnetism problems)
3) Insert left side decocker until the shaft is just touching the coil of the spring
4) Place thumb on coil of spring to hold in place while using index finger to depress trigger bar
5) pinch both side of decocker assembly with thumb and index finger of right hand- rotate simultaneously up 70 degrees
6) Snap decocker assembly together and rotate down 30 degrees- maintain pressure with right thumb and index finger
7) position the arm of the decocker spring under the leg of the decocker shaft - hold in position with left thumb.
Rotate assembly down into position- should click into place