Place a microphone stand in front of the cabinet.
Tilt the stand's boom -- the arm that holds the microphone clip and microphone -- until it is pointing down. The boom is typically adjusted by loosening a screw by hand.
Adjust the stand's height until the microphone clip on the boom is level with the center of one of the cabinet's speakers. The height is typically adjusted by loosening a screw by hand. Don't use one of the bottom speakers -- you'll capture too much reflected sound.
Place the microphone in the stand's microphone clip.
Move the stand right or left until the microphone is halfway between the center of the speaker and the speaker's edge. The microphone can be closer to the speaker's edge if you prefer.
Move the stand until the microphone is 1 to 10 inches away from the speaker grille. By placing the stand closer to the speaker grille, you reduce the amount of external noise that's captured.
Attach one end of an XLR cable to the microphone's XLR socket and the other end to the mixer or audio interface.