on mic-ing guitar amps, there are many methods. no right or wrong answer. I'm sharing what I know.
1. SM57 is still the most common 'go to' mic to go to for guitar amps. pointing it towards the centre of the cone will be more boomy as compared with towards the edge. a common practice is somewhere in between. there is some 66% rule if i recalled correctly. it means putting it 66% away from the centre of the cone.
2. The mic can also be angled. putting it directly perpendicular to the amp's speaker grill is known as mic-ing on axis. putting at a certain angle will affect the tone too. do experiment with it
3. As for distance, its typically between 1" to 10" from the cone. It affects the tone due to the 'proximity effect' microphones have. (imagine talking into the mic with your mouth touching it, itll sound bassy, while slightly further itll sound natural). The further it is away, the higher chances of 'bleeding' from other external sounds.
4. For studio environments, its common to add another condenser mic at a further distance to capture more of what the player is actually listening to. there are calculations for this especially if the speakers are in stereo (we typically dont want a phasing and cancellation effect). however, i'm not gona give a technical lecture here on the calculations.
i've learnt from mistakes i did and here are some common things we guitarist do. for those of you here who are starting to gig, well hopefully this can help.
1. avoid the smiley face EQ. human ears tend to be more sensitive towards mids (roughly the 3kHz region), therefore, the smiley face tends to give a nicer, louder impression (when at a lower volume). when gigging/recording, volume is usually high, therefore making the impression useless. what you end up getting is too much bass and treble and no mids. you cant cant through a mix without mids.
2. during soundchecks, guitarist usually setup to have their sound so big that it takes up the entire frequency spectrum. they sapu everything from bass all the way to the highs. its alright to use a high pass filter to cut off the bass. your guitar sound will not sound that good on its own, but once the whole band plays, its so clear and everything comes out nicely in the mix (with no cancellation or 'boomy'-ness) and overall, you sound much more professional.
3. we usually set up our EQ by standing slightly further from the amp. (i mean, hey, who actually puts their ears on the speaker when they play?!). remember that your mics are so close to the speakers. so if you actually walk and listen to your monitors/house system, there is alot more highs (thus you can sound really sharp on the house system). so after setting the EQ where you find nice, its always safer to slightly lower down your highs. if you are daring, put your ears next to your speakers and play and you'll get this point. (dont hold me responsible for losing your hearing though!

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4. putting your amp on the floor tends to sound more bassy and sound bounces off the floor and bleeds into your mic. try putting it at an angle (or on an amp stand)
as for what i do live:
usually a 2X12" is provided. if there are alot of instruments in the band (or there is more than 1 guitarist other than me), i'll just mic up one speaker.
if I am the only guitarist and I need a bigger sound, I'll mic both speakers, with each of the SM57 with different distance from the cone, and diff angles (one on axis, the other angled, usually about 30 degrees). i then pan the mics one hard left and another hard right. as the result, there are 2 slightly different tones coming out from the house system, giving it a nicer bigger sound.
hope it helps. thats all i can think of now. (gosh didnt realise i wrote so much). anyone got anything to add that i missed?
edit: sorry double post