Ah...yes. The 12R5 shock cap.
My best technique is to fill the shock with oil to just above the lip where the inside plastic ring seats. Pump the piston to make sure the reservoir is full and not trapping any air. Make sure the oil is bubbled out. Gently seat the plastic ring on the inside lip with the tip of a 1.5mm hex driver. A little oil should force itself around the ring. Lightly dab it out with the tip of a shop towel. Slip the o-ring over the piston shaft (I've long since caved in and just use new o-rings when I rebuild the shocks. It's so much easier, as they do swell after being in the shock for even a little while. Green Slime them. O-rings are a cheap consumable.)
The end cap: Hold it lightly so you can feel the threading. Lightly press it down on the threads and twist it backward. You should be able to feel a subtle click--maybe not even a click--more like a snick--when the start of the threads pass over each other. From right there, gently thread the shock cap on. If it's easy and goes right on, you should be good. If there's any resistance at all, don't force it! Just back it off until you feel the thread snick on your finger tips and start it again.
A properly built 12R5 shock lasts a long time. I like them a lot. I think the main frustrations are experienced when guys rush the initial build and cross-thread the end caps, or if they try to reuse swollen o-rings and, again, can't get the caps to seat properly.