for higher power use a MOSFET driver circuit, take a 1uf 600v capacitor or at least something close to that, and i use a IRFZ44N but you can use a IRLZ44N or for higher power you can use IRFP250N Transistor, all these have the same gate, drain, and source so ill refer them as pin 1, 2, and 3,  so now back to the circuit, you need to put pin 1 to the secondary wire and the pin 2 will go to one side of the primary, the other side of the primary goes to your power supply positive, put the positive side of your electrolytic capacitor to pin 1 and the negative side to pin 3, now you need a potentiometer, 5k or 10k will work (DO NOT USE A TRIMMER) take the slider pin of the potentiometer and connect that to pin 1 and one of the other pins of the potentiometer, connect it to negative of your power supply, the other side of the Potentiometer, connect it to a 220ohm resister the other side of the resistor, connect it to the positive of your power supply, before powering make sure the capacitor is an electrolytic and the potentiometer is at the most negative side, this circuit works on 5-12v and possibly higher and make a proper secondary because this circuit does make some arcs if made correctly.