It's best to use a dynamic microphone, such as a Shure SM57, to mic a guitar amp or cab for use with the Stream. If using a condenser mic, note that the Stream is not equipped with phantom power; you'll need a dedicated phantom power supply or preamp equipped with phantom power to use phantom power mics with the Stream.
Warning: if at any point during this process, you experience dangerously loud volumes, immediately remove your headphones and turn down the Stream's large volume knob. This shouldn't result from any of the Stream's functions when used properly, but with the wide range of possible signal chains and audio setups out there, there's always a chance of unexpected results.
Step 1: Turn your input gain and volume knobs all the way down.
Step 2: Start with your microphone placed halfway between the speaker cone's edge and the center of the speaker, with the microphone directly facing the speaker. Leave only about a finger's width of space between the microphone and the amplifier's grill cloth.
Step 3: Connect the microphone to one of the Stream's inputs using an XLR cable. As long as you're using an XLR cable to connect to the Stream, there's no need to adjust the channel's line/instrument switch, as the Stream automatically sets an input to mic level gain and impedance when it detects an XLR connector.
Step 4: While sounding the guitar, turn the input's gain knob to about 25% (9 O'Clock) or to about 1/4 of the way up and set your volume knob to about 50% (12 O'Clock) or about ½ the way up.
Note: Do not continue to turn up the volume if you experience unexpected loud volume or unexpected bursts and pops. Instead, check your connections and signal path and start over.
Step 5: While playing the guitar at your expected practice volume, adjust the input gain knob until you can comfortably and clearly hear your guitar audio. If you experience unexpected signal distortion, your signal may be clipping, in which case you should turn down the input gain knob until the clipping distortion ceases.
Note: If you're also singing, you may have to turn down your guitar amp volume lower than you normally would for a live practice session or show to avoid excessive guitar audio bleed into your vocal mic.
Step 6: Adjust the microphone position to your preferred tone. Moving the microphone closer to the amp's speaker increases bass frequencies while moving it farther away will reduce the bass frequencies. Moving the microphone sideways, closer to the edge of the speaker, will reduce midrange and upper-mid frequencies. Moving the microphone toward the center of the speaker will emphasize midrange frequencies.
Step 7: Adjust the volume knob to your preferred monitoring volume.