Why so vital? You could be playing your favourite song to the best of your skill in perfect timing to a drummer, but if your guitar is out of tune then it will sound amateur.
If you tune up every time you play then your strings will not go far out of tune so it will be a very quick and simple process.
Golden rule though, as you play the string the tuner will detect the note not the string that you are playing and tell you whether you are high or low. But if your B string is very low then the tuner may pick up an A note being played instead and point you to that. This can happen with all strings.
When the tuner hears the note it will show the letter or number of the string it has detected- make sure it matches the string you have played;
Fat-to-thin: E-A-D-G-B-E ( 6-5-4-3-2-1 )
Most modern guitar tuners have a ‘Flat’ function which when selcted aims the tuning target to flat. If your guitar has a whammy bar then you may have to go through the re-tune 2 or 3 times before the notes stop drifting.
Make sure that you use at least 9-46 'Hybrid slinky' weight strings if you flat tune regularly, the 'super slinky' 8 gauge will be hanging too loose and cause buzzing.
In my lessons I use the software 'Sound Forge' to raise the notes without changing the speed so that we can play to the song without going through the comotion of flat tuning all 6 strings.
Now your 3 low strings are automatically a powerchord when all are open or played over the same fret. Therefore the chord can now be moved up and down with just one finger flattened over all three strings; now D-A-D.
Because the powerchord is formed with just one finger not 3, rapid changing is much easier as you can change finger to finger for changes from chord to chord instead of moving your entire forearm. Example: Deftones My Own Summer.
Another use for 'D' tuning is that because the three open low strings let you play a D powerchord without a single finger, you can set off the chord with your first hit, let it ring on and play another riff/melody on the higher strings. Use this to make playing around with D scales sound fuller. Similar example: Nirvana All Apologies.
If you play a lot of heavy music in D then you will need a set of strings with a fatter bass string, I reccomend a set like 10-52 as these are tailored with an extra heavy E. I use these because although tougher on fingers, they give solid rich sounds and hardly any rattle when you go Whack.
This is used by Dark Metal bands with masses of overdrive. The deep and rumbly strings sound like your guitar is falling apart from the power.
Regular guitar tuners do not recognise the notes involved in C, so play the strings on the second fret but tune them as if they were open. All are then 2 semitones lower: C-G-C-F-A-D
If you use C tuning regulary then you will need to buy what is known as a Chromatic tuner. These pick up on every note of the scale rather than just those associated with standard E tuning.
For shows, make sure you fit super fat strings like 11-58 weight as a regular set will go so loose they sound like rattly rubber bands! But only use them in C, these will be tight as hell in standard E and will damage cheap guitars.
