The skill of tuning a guitar or any other instrument is the most important thing a musician need to learn. It does not matter how good you play a piece of music if your instrument is out of tune, it will sound terrible. Your instrument should not only be in tune during a performance but every practice session. Sometimes you will need to check the tuning a couple of times during a session. If you have new strings and if you play a piece that requires a lot of bending, the guitar will be out of tune pretty fast.
Tuning the guitar is usually very difficult for the beginner and it will take time and patience to master it. If you ask a guitar player: how long have you been playing guitar? The answer might be; I have been playing for 1 year but tuning the guitar for 9 years. Maybe it is an exaggeration but I would say that it is some truth behind the answer. In the beginning it will take you some time before you master the tuning but do never give up. You will soon see improvement and you are going to make it faster after every tuning session. To get your instrument in tune in an easy and accurate way, my recommendation is that you always use our guitar tuner online. However, it is very good to learn how to tune your instrument when you do not have access to a guitar tuner.
There are four main methods for guitar tuning; with an guitar tuner app, with an electronic tuner, with a reference of an other instrument (such as a piano) or with relative tuning, i.e. tuning the guitar to itself. We categorize these methods in tune with a guitar tuner and tune by ear. Regardless if you have help from a tool or not you should always use your ears when you are tuning. When you use your ear while tuning with a tool you will teach your brain how the strings should sound when they are in tune.
Remember that you always should tune your instrument in the same environment as where you will be playing. Most instruments are made of wood which is a living material. The wood tend to absorb moisture and will expand or contract depending on the temperature and the air humidity. The wood expansion respectively contraction will impact the string tension and therefore also the pitch.