In order to 'flash' a genhead the first thing you need is a rubber raincoat. The rubber is so you won't get a shock.......... Ha, ha.
Seriously, flashing the field is easy. It's purpose is to implant some magnetic flux into the core so that it can start building up field voltage when it is spun up. There is often enough residual magnetism left for these to work but sometimes it goes away. Shutting off the prime mover while electrical loads are still connected to the output of an alrenator is one way you can spoil the residual field.
You need to identify which of the two wires from the bridge to the slip rings on the field armature is positive and which is negative. This can be done with a digital multimeter in diode check position. Look on the bridge for terminal ID. You may find the positive and negative output terminals are marked. You want to lift/remove one of the wires that goes from the bridge to the slip rings, so that if you have a polarity error you will not let the smoke out of the bridge. Then with clip lead wires you want to connect a 12 volt car battery across the slip rings going to the armature field winding. The wire lead that was on the positive terminal of the bridge will go to the positive terminal of the battery. The neg bridge term to neg of battery. The connection should make a small spark as only a few amps flow. You need to hold the connection for only a few seconds. Voila. Field is flashed. Disconnect the clip leads to the battery and reconnect the slip ring wire to the bridge. It should work now if all else is well. If all else is well and you happened to flash the field in the wrong polarity it won't work. In this case reverse the pos and neg leads on the bridge output to the slip rings, OR re-do the flash procedure with the bridge isolated and the battery terminals reversed from first try. Put away raincoat optional.
