LEV, charging up a cap is simple, put the gnd terminal to gnd, and put the + terminal to the flyback, let it run for a second to charge, then even after you turn off the flyback the cap will still be charged, so take another ground wire and touch it to the +side to discharge it! (get ready for a bang!)
I used 1uF @ 4KV capacitor before i was satisfied, but make sure the cap can handle the voltage
BEWARE OF HV yayaya, touching a charged cap will zap you good, USE PROPER INSULATION GLOVES.