Attach your microphone to the USB interface or mixer and put it on your boom stand. It is important you use a boom stand; otherwise you won't be able to angle the microphone into the desired position in front of the amplifier.
Angle the boom stand so that it lies about an inch away from the middle of your amp. If you are using a single speaker amp, position the mic in the middle of the speaker. If you are using a two or four speaker amp, place the microphone in the middle of your speaker set up so that it will evenly pick up the sound for better quality.
Plug your guitar into the amp and crank up the volume. Start rockin!