Discharge the microwave's capacitor. Unplug the microwave, then touch one end of the screwdriver to one of the chassis terminals and slowly slide it to the second chassis terminal. Hold the screwdriver at the second chassis terminal for a few seconds. Expect to hear a popping noise.
For the high voltage capacitor in a microwave oven, use a 100K ohm resistor rated at least 5 kilovolts and several watts for your discharge widget, with a clip lead to the chassis. As a practical matter, a single resistor like this will be hard to find. So, make one up from a series string of 10 to 20 1/2 W or 1 W normal resistors.
the capacitor rating (uf) is related to the cabinet size the microwave, I believe the smaller the microwave the larger the cavity, but I could have that backwards.
A start capacitor kit contains a start capacitor, relay and wires. The start capacitor gives the motor's windings an electric boost during the start-up phase. The relay disconnects the start capacitor from the motor's electrical circuit once the motor has reached operational speed.
Microwave Capacitor: Test the capacitor by first discharging it. Isolate the capacitor and check for resistance on the most sensitive scale. Your meter needs to be using a 9-volt battery. Place your meter leads on the ends of the cap and you should see a quick rise and fall on your meter.
The wires will normally be changed one wire at a time or photographs would have been taken of the original capacitor in order to document the wiring connections. It sounds like you are past this point in the repair. If you take the microwave and the new capacitor to a technician, then you may be able to get the microwave repaired properly.