I just use a "clip lead" (a wire with alligator clips on each end) with one end on the chassis (make sure you pick a part that is connected to the CRT ground, or just clip to the CRT's ground strap/connection) and the other end on the screwdriver shaft. Yes, obviously an insulated-handle screwdriver.
I slide the screwdriver tip under the HV cap until it touches the metal clip. I also use the same grounded-screwdriver assembly to go ahead then and unclip the HV cap from the CRT. Once you did the discharge, it should be OK to lift the edge of the rubber/plastic cap to see how to unclip the wire. Once you have unclipped it, do not touch the anode terminal of the CRT, as it will "self-recharge" to some degree. Discharge/ground it again just before you reinstall the HV wire again after your repair.
I should also point out that a charged CRT, especially on a small tube as in an Imac, probably does not have nearly enough capacity to "kill anyone". I got zapped from a
plugged-in and live HV cable on a 23" B&W monitor chassis once. It felt like I got hit by a truck, but it was not fatal.
HOWEVER... Do not take this comment to mean in any way that such an activity is safe at all, and do not try this (touching a live flyback output) at home, or at work either for that matter!